from what you described i think you would probably like 4.8 x5 more. its true the ride can be a bit more harsh but looking at your other choices (m3 and s4), i dont think the x5's ride is worse then those, and x5 is perfectly fine for the type of offroading you are talking about. and since you want more performance, then the 4.8 might be more your thing then the 3.0. but why dont you go and test drive both cars and see which one you like more.

any all terrain tires are not as good in snow as snow tires. snow tires are designed for the snow, all other tires made sacrifices in their design to serve a different purpose and i might be wrong but i dont think any general purpose tyre could out perform a speclized tyre.