Well its 50/50 A lot of people who have larger reef tanks, put them into walls, and some don't. Personally, My 150 is flush up against my wall on all three sides. Had it built to fit a room. I also believe that Built on external over flows are basically the same as internal. Same method and build- Water flows over teeth, collects and travels down a pipe or dorso pipe into a sump. hang on packs have one drawback and thats the siphon, not practical when theres a power outage.
of course pipes can crack and come loose, leaks can develop. EVerythings has its ups and downs
so to say which is the best method- IMO Externally built/internal overflows are much much better then hang on siphon methods.