I have used both, and as far as filtration goes, they both do the same thing, just as equally.
Its really about ease of maintenance, and what is simpler to do for you.
If one makes maintenance easier for you than the other, and you do it more regularly, that's the best.
If you don't like bending down, or crawling around to clean media, then maybe overhead is better.
With my 300+ system, I put my sump slightly lower than the main tank to make use of gravity, but also off to the side so I can easily reach in unimpeded for maintenance with too much contortion, kneeling or even bending.
Its really about ease of maintenance, and what is simpler to do for you.
If one makes maintenance easier for you than the other, and you do it more regularly, that's the best.
If you don't like bending down, or crawling around to clean media, then maybe overhead is better.
With my 300+ system, I put my sump slightly lower than the main tank to make use of gravity, but also off to the side so I can easily reach in unimpeded for maintenance with too much contortion, kneeling or even bending.