So, frame mounts tend to be less abusive to the optic, but they put the dot higher, which lots of people say is a bigger learning curve if you're coming from irons. I don't think it matters all that much, but that's me. And I went to the frame mount after getting a bunch of time with slide mount optics.
Big considerations:
--Are there rules that would mandate you to have one over the other? Something like uspsa carry optics or 'stealth' division at some multigun matches is slide-mount only.
--Holster selection is another concern. Generally, frame mounts require a custom kydex holster if you want a kydex holster. Or you could go with a race holster that only covers the trigger guard. If you have a slide mount, you can get retention holsters if you want one.
Which all boils down to: what kind of matches are you going to shoot?
I love USPSA open guns, and I like the frame mount best in that context. I don't shoot enough multigun to comment much there, but for most matches near me if I'm shooting multigun, I just use my open gun, race holster, and the holster's lock lever if I need it.