The main benefit of a built in overflow is that they are much more reliable, assuming you are going to have a sump under the tank. A built in overflow will not be able to lose siphon, which could happen in a hang-on overflow. If the hang-on loses siphon, then your pump can run dry, burn up, and possibly dump a bunch of water on your floor. If you don't plan to use a sump, then you don't want or need a built in overflow.
Sonny