You wanna know the honest to goodness difference between the 500 and 800? Very simple, bragging rights. There is virtually nothing the 800 can do that the 500 can not. Granted the 800 has a whole bunch more torque the 500 will still turn 28″ mud tires, larger with a clutch kit, and get you out of a hole. The 800 has the obvious edge in lugging power on hills and such but we are not talking huge gains unless your an extremely aggressive driver. While I had a 500 HO and loved it I’d still opt for the 800. I like bragging rights, lol.
Reality is if your going to do your average trail riding, hunting and fishing, and don’t have to have the biggest brute on the block, the 500 is tough to beat. Get the 500 EFI and you likely have the best ATV on the market. Fuel mileage is a direct measure of the fun lever. My carbed 500 HO could get up to 23 MPG and my 700 EFI does the same with more power. However if I start to twist it’s tail it starts to get thirsty in a hurry. I can ride 85 miles with no worries on my EFI and could go 100 on my 500 as it had a larger tank.
If that doesn’t help just flip a coin. You will not go wrong either way IMO. One word of caution. If you’ve not ridden the 800 class machines before please understand they are not always the best starter ATVs. These are serious quads with serious abilities one of which is the ability to make you lay on your back with it on top of you. That normally hurts! Please be careful no matter what you choose.