For G15, we assembled on the pier after the tender ride, walked a block or so down the wharf to a small dock and got on the boat.  The reef is, literally, right there off of Georgetown.  It's the reason the ships all have to anchor out.
If you're a decent swimmer, both are fine.  For the G15, there's much more to see - tons of aquatic life, an enormous reef, the sunken ship and so on.  The G76 had the 'shock and awe' of these enormous and beautiful stingrays. 
For a first timer, I'd recommend the G15.  Tons to see.  TBH, I'm jealous, I wish I was there.  Our last Western, GC wasn't open due to COVID so we missed our snorkeling there.