With me not knowing anything at all...my dealer said "If you want speed, get the 800. If you want to just go on trails and have fun, get the 500". I weigh about 180. My old Polaris 325 was more than enough power for me, one person only. I'm still too new to a side-by-side to make any judgement.
For the jerking on take off, I don't have that problem at all. It is pretty smooth. It seems sluggish, but the dealer says that is normal.