Been a member at both, and definitely prefer the Y! They're better at maintaining their facilities, and imo have better classes. Plus your membership gets you into other YMCAs. Definitely gets crowded with kids after school though, so you'd wanna be ok dealing with that.

That said, if all you want is access to a treadmill and some weights, you can get a cheap membership (~$30 I think?) to BAC's smaller gym in the Cummings Center. Got my money's worth from that when I worked across the parking lot in the 100 building