If you have the room outside, go that route.
A. Inside of the tank will have clean lines and no obstructions to flow.
B. If in the event of a fish/snail go over the weir, much easier to get it out.
C. More room for plumbing. Most corner overflows will only fit 2 pipes, whereas the external you can set up a quiet running Bean Animal style drain.
I have had both. One of the must have's when I did my new build was an external overflow. So glad I went this route. My tank is dead silent.