Is there something specific that tipped it to UCLA for you in the end? I find it really interesting how people choose between two or more really great options, especially because mine didn't come down to anything concrete like cost or specific programs/opportunities, etc... internal stalemate until I got that gut feeling and ran with it. (edit for clarity: my 2nd option wasn't UCLA, it was Vandy, but I empathize with the toughest-decision-ever thing)
Well, there was a definite financial difference, which was a pretty big deal, especially since both schools offer very strong prospects. At the end of the day, I just felt better about UCLA and felt at home in Westwood. I liked Michigan a lot and thought everyone I met there was first class, but it didn't quite feel as much like home. I'm probably giving up a little bit in terms of career opportunities to go to UCLA, but I think it's worth it to follow my gut. Since I'm probably going to want to work in either California or TX/OK, Michigan's advantage over UCLA was a little less than it would be for most other applicants, and the money gave me enough help that I felt good about going to UCLA. If I had been faced with the prospect of paying sticker at both places, I probably would have had to go to Michigan, since I liked it there and it would more or less guarantee me the ability to pay it off.
Michigan actually offered me a very nice need based package. It wasn't nearly as much money as UCLA ultimately offered between need and merit money, but it was a very good offer that made me think very hard about Michigan. Like you, it wasn't an easy decision and it didn't really come down to any one factor: it was just a "feel" thing. I can't say that I would recommend the same decision for anyone else, but I do think my experience highlights the importance of seeing the schools for yourself and making a decision with that in mind. If I had decided to go to Michigan, my experience there would have left me knowing that I would have a really nice time there. My UCLA visit left me feeling like that was the place for me to be, and I'm really happy that the nuts and bolts allowed me to do that.