As a painting contractor, you have to remember the environmental impact of coating and recoating wood.

Wood siding should be painted every 5-8 years. There are different grades of fiber cement.  Assume about the same time frame or slightly longer than wood.

Aluminum needs painting purely for aesthetic reasons (the paint wears off), but only needs painting every 15-25 years.

Vinyl is the same as aluminum in that after a long, long time, you'll want to paint it because it fades.  But it can eventually dry and crack, so it doesn't last as long as aluminum.

Whatever you do, don't replace your aluminum siding.  It paints great, the coating lasts a long time and the siding itself will last longer than a lifetime.

Donna:

If you have "a bit of rot" it might be just the tip and could spread over time; if your house is part of a development ask around to see if anyone else is getting similar symptoms.  One advantage of re-siding is you can take the opportunity to upgrade your insulation and/or house-wrap, which can have a dramatic effect on your heating/cooling bills.  As for new siding, I have a predilection for fiber-cement, as I expect it to last a lot longer than 50 years, and it's repaint cycle should also be much much longer than wood.  I (like Rich above) am a painting contractor, and if you do go with wood siding, you should A start with bare wood (not pre-primed, it is garbage primer) and B have your painter back-prime every square inch, and make sure the installers seal their saw-cuts.  Do those and you should never see the likes of me again!

I am reminded that although there is greater maintenance, the wood siding on my 100+ year old house has never been other than patched and painted. Of course on a house this old removing the wood also probably means having the hazard waste since there is lead in the old paint. Unfortunately, someone prior to me covered covered it with vinyl and so I feel compelled to leave that until it cracks and must be disposed. I have checked under the vinyl siding while doing a couple of repairs and the wood siding needs painted but is still in otherwise good repair. Should it need to be repaired in the future, as any siding may need to be, finding a match is far easier than any other product since wood is cut to size and the pattern is not phased out of production with time.

Do you really HAVE to paint wood?  Isn't it just as well off if you don't paint it?  Can someone speak to this without bias?  

For timhammond - yes, indeed, you really have to paint (or cover) wood.  Otherwise you risk rot on the wet hand, or warp on the dry hand.  Some wood, like cedar, changes from reddish to grayish over time, but is quite expensive.  Log houses aren't usually painted and provide their own insulation value, very expensive.  

Have you ever looked at a barn closely?  Even they are protected with a coat of paint, but older and neglected ones turn gray with time, the planks dry out and split, pulling away from nails and fasteners, and then fall down.

Lapped wood siding looks nice, with planks that can be replaced if necessary, and so does 'board and batten.'

It all depends on the wood.  Some woods are extremely rot resistant and can last quite a long time without being painted.  Cedar, white oak, cypress, mahogany and ipe/ironwood are all examples.

Other woods will rot very quickly if not protected from the elements.  White pine (commonly used as siding here in the Northeast) is one such wood.

I personally wouldn't side my house with wood and not put some sort of treatment on it.  Warpage is a major issue with wood ...if one side of an unfinished board gets drier than the other, the wood WILL warp.  Heck, even putting a finish on one side and not the other can cause the board to warp.

I think the thing that really should have been addressed here is replacing the old windows with new ones.  An old wood window, properly maintained and with a good fitting wood storm window, is almost as energy efficient as a new replacement window, and will CERTAINLY last longer.  Any window you buy today will last 20 years tops before it have to be completely replaced, and you'll never recoup the cost of it in energy savings.  Old wood windows need repainting every 5-6 years and reglazing every 20-25 years to keep them functioning indefinitely.

If you want an impartial professional assessment, have a licensed general contractor, home inspector or well-recommended handyman/woman check your siding, not a siding contractor. Siding contractors are pretty much in business to sell re-siding jobs whether you need one or not.

Be sure to deal with the condition that caused the rot in the first place. Even a rot-resistant siding replacement like Hardiplank can be insufficient to protect the framing below if the detailing is wrong. Rot in the frame is much more of a problem than in the siding, though they often go together.

Wood grows with a protective skin called bark which is necessary to protect it from infection, infestation, UV degradation and excessive moisture. Siding is "flayed" wood, and not in its natural condition in any sense. Paint it to protect it.

I agree, the life of a decent-quality fiber-cement siding properly installed and detailed should be much much longer than 50 years. It's mostly concrete, the material of the Pantheon in Rome - two millenia and still going strong. And in my experience the paint job will last at least twice as long as on wood.

Actually, siding such as Eastern White Pine shingles can last for over 100 years with no paint, ever.  Which is not to say it's a bad idea to protect them, but linseed oil might do just as well, and doesn't look as ratty when it needs to be redone (of course, your house looking ratty may be just the reminder you need to re-do whatever coating you use).

Wood siding lets your house breathe.  I would be very careful about encasing a house in products that don't breathe (including new insulation).  Yes, it seems like a good idea to insulate and then cover it with something durable and low-maintenance, but if in so doing you have created additional vapor barriers, all you've done is move the potential rot to a location where you won't see it until it's far, far too late.  Most heat loss occurs through improperly maintained windows (including crappy new "replacement" windows, as well as older windows that need to be reglazed or have new storm windows installed) and through uninsulated roofs/attics.  

So blow some insulation into your attic, reside the house with sustainably harvested wood, and call it good.

Oh, and as for fiber-cement products lasting as long as Roman cement structures?  Yeah, not so much.  Check out mid-20th-century concrete (bridges and such) and see how "indestructible" concrete is.  Unfortunately, the one type of siding that does seem to be completely impervious to the passing decades is the fiber cement with asbestos that was used in the early 20th century, but the stuff they're making today is unlikely to be that durable, if you ask me.

There's plenty of 19th and 20th century concrete in first class condition and of course there's plenty of rubbish too. Every age has its share of crap; older stuff (including Roman) that didn't make the grade vanished long ago. We only see the survivors. So far I've seen Hardiplank go fifteen years on one coat of paint and still look great and perform well, and that's as long as it's been in use in my area. I'm professionally involved in a lot of renovation work and I've yet to see any failures. I don't expect it to last less well than the asbestos shingles you despise. Time of course will tell.

Fiber cement siding is not a vapor barrier. No lap siding is. Agreed that you need to pay careful attention to vapor barrier placement, especially in regions that experience prolonged cold weather.

Agreed that split shake siding can be very durable if that's what you mean. Sawn shingles and sawn lap siding are generally much more vulnerable to decay as the face grain fibers are ripped open by the saw and absorb moisture like a sponge if not protected. Good roof overhangs and proper flashings are essential in any case.

Agreed that poorly installed or poorly maintained windows are major energy sieves. Fix 'em. There are crappy modern windows and also excellent ones that will last several lifetimes if properly fitted to your house. You generally get what you pay for.