Asking which place is better is liking asking Florida football fans if Florida or Flordia state has a better football program. It's really a "homer" question.
IMO, if you want wrecks and artifical reef systems, I would think Ft. Lauderdale. There are wrecks to dive in Key Largo, but nothing like that stretch from Miami on up (just check out the web for dive sites, amazing). YES, there are nice natural reefs along the east Florida coast. Repeat...yes, there are natural reefs along that coast. However, if you want an abundance of natural reef systems, warmer water and the chance that viz could hit 80 - 100 ft when wind and current are right, then choose Key Largo. Remember, the closest points to land for the Gulf Stream are in the upper keys. Big fish? last November there was a 600 lb. Goliath (PC term) Grouper on The Eagle, a wreck out of Islamorada.
As for after the dives, I love Ft. Lauderdale but you just can't beat the easy pace and relaxing atmosphere in the upper Keys. You just feel like you're in the tropics.
Whatever you choose, you absolutely can not make a wrong decision. Ft. Lauderdale or Key Largo? I think that's a problem we would all love to have.