Bottom line is if your just a trail rider the 600 is more than enough. If you need to be the 1st one across the lake, then you need the 800. If you like to play in powder the power of the 800 really is nice. My main sled is an 800r Gade for off trail play. I am looking for a trail only sled and I can tell you that will be another 500ss. I have one for the girlfriend now. With some good clutching I made that little 600 spank newr 600 EFI sleds and keep up with 800s in the trails. Across the lake is another story.....
Fuel mileage is the same between the 800 and 500ss (16-17). The Etec will be a bit better, but not the 21 mpg they claim. If you ride it hard it will be just as bad as the rest of us.
If the cost of oil is a concern, to be honest its CHEAPER not to have an Etec. That sled you need to run the BRP oil at $35 a gallon. With my carbed sleds I run Sea and Snow for $13 a gallon. I may burn twice as much, but I still save $9. Also cheaper to work on your own sleds, which is carbbed models. You have a fuel problem with an Etec, you go see the dealer for $80 an hr.