Valletta is amazing, so much rich history. I spend my final day there after finishing a load of dives.
Gozo is meant to be much prettier, I've only ever been to the Blue Hole/Azure Window but m parents hired a taxi driver for the day as a tour guide and really enjoyed it.
Am contemplating going there for a year once I finish Uni, working at Diveshack for Rita and maybe a few months on Gozo too