Holy drama Batman. Depends on what you want I'd say. I haven't driven an X3 so I can't really judge, but if that type of vehicle appeals to you, I'd say it's pretty much a tie. We recently picked up a 1 series coupe, so pretty different priorities. There was no Mercedes that could match the driving experience. The only thing that drove as well (compared tons of cars) was the FR-S, but it was a hassle to get in and out of and kinda poorly equipped.
Honestly though, I have zero brand loyalty. I think it's silly and naive to like a brand. I like individual cars and evaluate them on their own merits.