unless you have a very small yard in which you wanna make an RC track, you should go for the 4x4 stampede as it will drive almost anywhere.
the others(mini, 1,18 scale) are only fun (IMO) on small RC tracks. in open spaces, they feel tiny and slow. on grass, they suck.

the 4x4 pede can drive in mud, snow, grass, dirt sand ect. but then you get what u pay for, as the pede cost alot more.