here is what I think for what it is worth. A recirculating skimmer, which is an external skimmer, is the most effective design. naturally there are advantages of each, but the external uses one pump to supply water and another to inject air. it leads to a longer exposure and allows the skimmer to work better. that said, I use an in sump skimmer because it is contained in the sump if anything goes wrong.
the height is another way to increase the amount of time the water is exposed to the skimmer making it more effective, so if they are equal in other ways the taller skimmer will pull more waste out.
the advice I see is to run a skimmer rated for twice the volume of your system. my system is 150gal so I run an 8" octopus skimmer rated for 300gal. I also know that you can over-skim. if you over do it, you can strip your system of trace elements, and also clean the water to the point where it is actually harmful to soft types of corals. I once over skimmed my tank while trying to start with softies if you look back at my old posts. it didn't work so well. stick to the guidelines out there and I think you'll find reefers quoting them because they work.