When I was your age I was riding a mountain cat 600 out west...but that was 7 years ago and I was new to mountain riding. since you already ride a viper, I'd say get an M7/8 because they are really easy to ride handling wise. I just rode the XP for 4 days and was pretty impressed with it (lots of power and super light) but I'm not sure they are a smart buy right now because of all the issues people are having. The air intake on the XP also likes to plug up and choke the motor in deep stuff. Don't get an 06 RMK, they will be a hand full for you to carve with (I rode it whe I was 18 and 5'11, and it gave me a work out) not saying you can't do it, but there are better choices that will be more enjoyable. I guess the new RMKs have made some improvments in the last couple years, but I haven't ridden them.
Since your still pretty young I'd say don't be affraid to get a shorter track, like a 141 or 144. They are a lot more manuverable and IMO you won't need the floatation of a 150"+ track until you get bigger and heavier. With a 144 at your age I think you'll still be able to climb with anybody and you won't get as tired as you would trying to man handle a big pig like a 151"+