Personally, I would NEVER run an external skimmer... for one reason...
OVERFLOWS
Some time you are going to add something to your tank that makes the skimmer go nuts. If you have an in-sump then the overflow goes right back into the tank. If it is external, your overflowing water goes onto the floor. Then the sump volume lowers and the return pump goes dry... overheats... Now you have a wet floor and damaged equipment. OR your auto top-off kicks in and your tank is flooded with freshwater and all your corals die.
Things that have made the skimmer go nuts include:
Epoxy for a frag
Red-slime remover
Some types of new "foods" (mostly coral foods)
Various additives