I'd really stay away from the X3, mostly because the ride quality is just too firm to be remotely comfortable, and it's feeling very long in the tooth now. The interior has quite a lot of cheap, hard plastic (although the minor facelift that brought in the updated engines did address this a bit), and controls from the E46 3-series. The X5 will also seem quite dated (I assume that for 20k GBP it would be an E53), since it's old 5-series switchgear in the cabin, but it generally feels much more substantial and well put together.

Anyway, that's just two cents (or pennies...) from a newbie member here.