X1, X3 and 3 Touring (3T) I've looked at. I'd say it like this: X1 has more headroom than the 3 touring, X3 has the most. I really, REALLY wanted to like the 3T but the engine was a complete disappointment - lethargic and initially slow (now, that said, it was unclear whether it was in eco mode, or something else, but I know I tried switching it and didn't feel any differences). Throttle tip-in I had to push a fair bit to get a response. The X3 has the 35 engine, which is such a huge step up from the coarse 28. I didn't bother driving the X1 because it was even lower on power and because of trunk space. Fuel mileage seemed to be a wash in any case - not much savings either way unless one compares gas to diesel.
Trunks: I looked at it from the perspective of carrying large suitcases. The X1 had a tiny trunk (as an aside, I thought the Mini Clubman utilized it better as it was taller) and it was short - really short. The X3 was a bit longer but shorter than the 3T. Where the X3 won was in the height. In the 3T the suitcases would be taller than the seatbacks if placed on their sides. The advantage of the 3T is that one could put a Thule on the top and it would be easily reachable for those times when you need the extra space. Back seats down, it was pretty close in terms of cargo capacity.
Interior: First the front seats: The X3 was the most comfortable by far, I could move my legs without colliding with the dash or tunnel. The X1 and 3T were too close to say one was better than the other but they were different in respect to the way you sat - the 3T was more stretched out, the X1 more upright. All had the sports seats. The big difference was in the back seats. The X3 was much more roomy and comfortable. The 3T I felt ingress/egress was a bit tight and headroom was pretty tight for my 5'8". The X1 felt narrow and small. Putting 3 passengers in the X1 or 3T would be tight.
Drive: the X3 was more supple, the 3T tended to rock more with the road undulations. Both had the adaptive dampeners. I honestly felt that the X3 felt smaller on the road, than the 3T. It was also easier to see out in all directions in the X3.
Given I've got growing kids, I'd probably do the X3. If it was just myself, then the 3T (and I'd lose some weight). I wish they offered the 35 engine in the Touring over here.