I think tyre choice makes the biggest difference in terms of not getting stuck followed by ground clearance(moorland access means deep ruts?). You would be amazed what even a 2wd vehicle can acheive with the proper equipment.
If you're just putting it in a muddy field then anything with an "awd" will be fine I'm sure. If you're going off-road in the "proper" sense then get the higher car.
