Rakins 800,
Go with the 144 conversion and pick up a used cobra track.
I used to be a grade runner, the kind of guy that stays on the trail when my buddies are having fun in the power lines. Yeah I had fun smoking cigarettes and talking about how fast my snowmobile was, but I missed out on fun stuff.
So I went from a 121 on a 2010 IQ shift to a 144 with the tracks usa extension kit and a cobra track bought from someone on this website. Man, did that make a big difference. It didn't handle quite as well on the trail. It pushed a little bit in the corners, but overall was a better snowmobile
I totaled that sled ditch banging in Carey, WI. My cousin nudged me into the sticks and I had to claim it. Now I have a 2012 Assault 144. Wow, this thing handles well. Like it's on rails.
Long story short, you'll like the extension. Go with the 144 you won't regret it.