I have been racing SnoX for 3 Years now and just riding sleds in general my whole life. Everyone seems to have a favorite brand that they stick by. The fact of the matter is they all work, and the differences in performances really aren't that great and can be made up by a good rider.
I'm not sure what elevation you plan on riding at but in Alaska we have a lot of terrain that is at a lower level so you can get away with a smaller cc engine, as some of you may know as you go up in altitude you lose horsepower. I would recommend getting a 600 if you have never ridden before as they can certainly go places you don't want to be and will develop you as a rider so you can handle a big bore later on.
I haven't spent too much time on the newer mountain sleds since I have been racing so much but I did get a chance to play around on a 800xp summit back in march and I must say I was really impressed, it floated extremely well and was remarkably stable, I thought the side-hilling was okay not too impressed with that though. Overall a great sled. I was riding an older 2002 800RMK the same day with the same length track and there was no comparison. I have ridden the new IQ RMKs, M series, and the Nytro MTX. I liked all of them. My advice is to go demo each of them and pick what one feels best to you. Don't be afraid of the 4 stroke weight issue. If you mainly want to go backcountry so you can ski I doubt you will find a more reliable turnkey machine then one of the new yamaha's. I have a personal preference to Polaris since it is what I race but I'm not oblivious as to what the other manufacturers have to offer.
If you are going to go used, the same advice applies. Take your time and try all the different models, find what you like but make sure to check for the standard mechanical things. 1500-2000 miles is a lot for a mountain sled because it is usually a lot of wide open throttle running. Cracks in the rear suspension components, tunnel, bulkhead or nuns especially if your looking at revs. Bearings, if it has been greased, ask about the last time the shocks were rebuilt, and other general maintenance items. Buying a used sled can be pretty tricky so definitely be cautious and don't jump on the first sled that has an exhaust and raised handlebars.
I'd be more than happy to answer any more questions in this thread or PM's.