True the Tunze DOC skimmers are real nice.
Yes, the Coralife and AquaC both have their bodies on the outside of the tank and the pump hangs just on the inside of the tank, not on the bottom. So no footprint. Just need enough space between wall and tank.
Surprised why I haven't asked yet, but what size tank do you have, is a reef tank, and do you have a high bio-load? These can all be factors in your decision as to how"great" of a skimmer you really might need.
If it's just a FO with a few softies, a religious water change regimen can do just as good as a skimmer.
A reef tank with lots of sps and fish will require a larger, and probably more expensive, skimmer.
If you can run external, then do that. You will transfer less heat to the tank and you can practically put it anywhere you want it. Be careful with "modifying" skimmers to run external when they aren't supposed to. If you have to mod it out of the box, then you probably should have chosen a different skimmer.
Also, if you run an external skimmer that's not meant to be, then that usually means running a pump under pressure ona pump that isn't pressure rated.