First of all I have rode a 155 and 163 iq chassis and all I have to say is that the 163 when riding in the trees you don't have to be going balls to the wall because your floating better then the 155. I got the 163 pro 2012 and its even easier too ride then the iqs.(I know everybody knows this)...
Just ride what makes you comfortable and you like. I love my pro. Now if i had to ride a ski-doo, give me time I would adapt to it. I am a good enough rider, they are a good sled. The thing is that the ski-doo shop where I live don't know their head from their ass when it comes to...
Handleing wise no difference between the two imo, the 3 foot powder day I left the 155 behind. Very glad I got the 163. I still jump and do every thing a 155 will do.
Put it this way you need to ride what makes you happy, if we all rode the same sled it would be pretty boring. But on that same note my thoughts exactly on the ski-doo's, they will not sidehill too save your a$$, my 85 phazer feels more comfortable then the ski-doo's. They are a good sled, but...