Hi rjdm,
Both are very long boat rides with open water diving & mooring. Can be rough trips. Both have coolish scuba temps & sometimes very demanding currents/surge. Neither is appropriate for the inexperienced or unconfident diver.
Cocos typically has more sharks, whalesharks & cownose rays, while Socorros has the world famous friendly Pacific mantas (pretty hard to beat this experience) and humpbacks in season.
Cocos liveaboards are decent craft, but nothing special. The Socorros Solmar V is a lovely vessel, although the cabins are smallish even by liveaboard standards. The newest Socorros edition, Nautlus Explorer, is not as pretty, but has a little roomier berths.
Nice thing is, either choice you should have super diving.
Regards,
DocVikingo