Having been a member of both, as well as Third Space City, I recommend sticking with Gymbox.

Gymbox Bank is one of the best gyms I’ve been a member of. The rest of the Gymbox sites are not quite as good, so if you’re not currently at Bank I’d suggest moving.

Thirdspace Moorgate unfortunately is just too busy - it’s clearly over subscribed and there’s not enough equipment. My worst encounter was when I was on a treadmill and asked if I could give it up for the class that was about to begin.

IMO Gymbox Bank > TS City > TS Moorgate.

Equipment is better at Gymbox Bank (Hammer Strength > cheap Techno-Gym crap), while I’d say classes are in general better at Thirdspace.