Some friends and I have been planning a three-year round the world trip that we want to do in a decade or so. There is alot more planning to go.
With regards to tents, when you are out for that length of time you not only want something that is overall bombproof in the nastiest weather, but that each independent part is going to last aswell.
At the moment we are figuring that we will likely use a pyramid tent. It will likely be homemade, so that we can get the exact dimensions and features we require. Material will probably be something like 2.20z ripstop silnylon, plus reinforcing patches, like a small rim of extra fabric along the bottom. We will have some mesh skirts for insects, multiple vents on the top, possibly a chimney hole (for a stove like a Kifaru in Russia and some other cold winters), and many guyline tie out points. The capability for internal guylines, clothes lines, hanging mesh pockets, etc will also be factored in. We are also thinking of having the top foot or so made out of a gore-tex or preferably Event fabric for extra breathability. The tent needs to be flexible in pitching options-will work where to have guy out points so we can use it as a lean-to/tarp in milder climes.
The main negative to a pyramid tent is of course its ability to freestand. This can be a problem in rocky areas, but I have found in South Australia (and some particularly rocky campsites) that you can usually make do and find a way. I am yet to use a pyramid tent-I used to use tunnels alot which have a similar, but less critical problem. I think that having a few lightweight (in reality rarely used) silnylon bags that you can fill with rocks, soil, sand etc, would help you get away with it in the worst areas. Also, having spare cord that you can attach to peg out points can make a difference-you can run it along the ground to where you can peg/tie it off. Another option we are considerin is some simple pole sleeves/velcro attachments for the bottom edges. With a four to six sided pyramid (another factor yet to be decided) a few lightweight CF or alloy poles could be threaded into these sleeves to aid in free-standing the tent-its not perfect but would help.
While there are some incredibly strong conventional (dome, geodesic, etc) tents out there with good components, we feel that a pyramid will last longer over that length of time. Its easier for us to repair, it only has one bomber straight pole (which we can replace with just about anything), and is floorless. We will have simple groundsheets aswell as probably some single person mozzie shelters with tub floors for inside it.
If you want to get a conventional tent, look at detail at how strong the contruction is. Particularly poles. I have noticed the latest range of Mountain Hardwear Expedition tents have some awesome features on their poles which make them easy to repair, very strong and certainly expedition worthy.
Remember you may not be able to find replacement parts or get your tent sent away for repair in all the places you are going-and if you can you will possibly be inconvenienced in the mean time.
Hope this helps; its another way of looking at things.
Good luck with the trip-I look forward to hearing how it goes-it will be good inspiration for me and my friends.