Welcome to Flyertalk. You probably know this already: T5 itself is divided into terminal A, B, and C, connected by an underground train. Each of the A, B, and C terminals are themselves of a decent size. Longhaul flights would usually park at Terminal B and C, and shorthaul flights usually depart from Terminal A. To transfer between your flights, you will probably have to walk to the underground train platform, wait for a train (sometimes it could get so full that you won't be able to get on the first one), and clear security at Terminal A. All in all, the minimum transfer time requirement within T5 could not be much less than that between T3 and T5. So I would recommend the AA flight, even just peace of mind.