There's no replacement for displacement. If you want to hang with another SxS with an 800cc, it's always advisable to stay close to that mark when you buy. Now, having said that, be prepared to pay a lot more to feed that thirsty engine. I'm going to guesstimate here, but expect to get 20mpg from the 800, and up to 30mpg from the 500. That's a big difference, and only YOU know where you will be riding. If you routinely put 75+ miles on a machine over a ride, then the 500 may be a better choice unless you want to invest in Rotopax tanks to add to the 800s overall range.
Cosmosdan hit the nail on the head !!! Stay out of the mud, its get very expensive very fast. It trashes bearings, tears CV boots (submerged sticks, bottles, etc) and shortens the service interval greatly. I can guarantee you that after I buy a Zforce 800, the last place I will willingly go is through a deep mud puddle.