I have both. I had a 75 originally, but sent it back when I got a helluva deal on a 55 and a MFT1080 and an OF 1400 and a BUNCH of accessories for barely more than I paid for the 75 (divorce is very hard on some guys

).
I then got a 75 used for a bit of a deal after having used the 55 for some time. I use the 55 for light duty work on the MFT, and for sheetgoods. If the wood is even a little on the thick side or hard, I bust out the big dog.
I know I am very fortunate to own both, but if, with some experience, I could only get one, hands down, it would be the 75. It can do everything the 55 can do easily, and MORE. It is a bit heavier, but I'm not bench pressing, it, I'm laying it on a guide rail and pushing it. If that gets hard, I spray one some Top Coat and try again. The price difference may seem high up front, but longevity is less expensive (in my experience.)
I love both saws, but if I had to give one up, it would be the little brother.