Adam,

The important thing to note is that the 4x4 on the Polaris doesn't engage until the wheels silp 1/5 of a turn. If you are just doing mild trails, then why would you want the expense of a 4x4? A 2x4 will pretty much go where you want to go. The only reason you would need a 4x4 is serious mud. Since the Scrambler 4x4 doesn't have a dual range sub-transmission, you'd probably just end up burning a belt on it in serious mud holes.

Stock for stock, the 400 and 500 are about even in acceleration and top end speed. If you modify them to their fullest extent, they will still be about the same. But, for a dollar for dollar basis, the 400 is a LOT cheaper to modify than the 500.