External will give you some more space inside the sump, however will rob you of space for things like a chiller and reactors. Internal robs you space inside the sump but leaves you with plenty of space for chillers and reactors.. Really it depends on what else your going to have in your dedeicated sump space. Also there is no real performance advantages to one or the other (besides brand). A lot of people really like the Bubble Magus skimmers. I personally use Reef Octopus skimmers which I think are great, but a bit more on the expensive side when compaired to the BM.