I've been on a 154 all season. I'm up to around 1,500 miles and absolutely love it! Well it's down right now, so I decided to take out the shop 165 demo. The conditions were possibly the deepest of the year. Three to four feet of blower pow on a solid base. The 165 traction was impressive. It pulls and pulls. You don't have to be nearly as quick on the throttle or as quick with body movement to get the sled to do what you need. On the 154 little body movements and english will make a huge difference in what you can get the sled to do. The 165 wasn't as easily effected. It defiantly felt bigger and longer because it is. I know some say you cant feel a difference but I could. The 165 climbed up steep and deep terrain with less effort for sure.
Personally I'd rather be on the 154. It has better track speed and just felt more agile in the tight trees. We ride very tight trees in our area and the 154 is better for me doing that. If I lived in the BC area where you have tons of open space and deep deep snow all the time I'd likely get the 165. When conditions are average to sub or in tight trees the maneuverability of the 154 is my choice. It's also what I'd pick for whips, re-entries, and popping pillows in general. I can whip it around in the air with the track rotating in such a small space.
The last thing I'll address is the suspension. The demo I took was what you get from doo out of the crate. I've been on full Raptor shocks with the t-motion locked out all season. I'm so used to it I really didn't think much of it. Well let me tell you...It's a huge difference! The stock suspension felt like a big wet noodle compared to mine. It was harder to control, was never stiff enough in the skid, and bottomed out too easy. Good suspension is expensive. I know some guys are over it and feel like you don't get what you pay for, but with my experience it's well worth the money. I would really have a tough time going back to what they give you from the factory.