I personally used the Uni gym for the year and didn't really have an issue with it. I used to go early when it was quiet then on to a lecture or library or whatever. It's £135 I think, for the year, and I tended to go a couple times a week.
Simply is also good, but there always seemed to be something out of order when I was there (to be fair, I went only about three times in the year). It was also always busier than the Uni gym, which is funny considering what above posters said. I think it depends a lot on when you go.
But, as people said, Simply is cheap and that's a pretty compelling factor lol. There are separate rooms too and it is certainly bigger than the Uni gym.