You guys are funny. He asks for facts on reliability issues and you give him weight differences.
So to answer your question to the best of my knowledge, yes the yamaha 4 strokes are very reliable. Never heard of any problems nor have I had any in the past 3 years riding them. Paying an extra $2000 for a sled that is 3 years newer is the better way to go no matter what sled it is given that it is in equal or better shape. Besides worrying about engine issues, a 3 year older sled will just plain old have more wear on everything as a whole.
Now I can go into the weight issue. Yes of course the RX is heavier. I'm about 160 lbs and I rode a warrior for 2 years. The biggest problem that I ran into being the weight I am is sidehilling in powder. Its a little tough to get it stabled to climb out and back up. I got it done no problem but thats where I felt most of the weight. Besides that if you aren't a ***** you can manoeuver it anywhere else without a problem. And it does have gobs of power.
The MXZ 700 is a reliable sled as well. My buddy has been riding the same one for 4 years. Never any problems with it that I know of. I think he has 8000 km without touching it so far.