Do you want to enjoy the outdoors?
I ask that question because I really feel that a Class B would be great for a one-year trip. At least for me. I'd get the screenhouse I mentioned in a previous post, preferably one with the option of also being a full tent with zip-downs to reveal the screens. This would give a place to stretch out and have some privacy when you're stationary for a few days. I see too many people with bigger rigs who never get outside. Many...certainly not ALL...seem to spend their time camping inside their big rig. Doesn't make much sense to me.
I think it depends on how much gear you think you need for a year. For me, I would go minimalist just to make things easier. But I realize some folks might not like that. Also, it depends on where you'll be traveling and at which times of year. If you are going to keep to warm climates in the winter and save the more northern areas for winter, again I think a B would be fine. But if you want more flexibility while traveling and might find yourself where it's cold/wet for extended periods of your trip, then the bigger C and A rigs would probably be a better choice.
Just my 2 bits and, on this issue, probably worth less than that! :)