I have a Super Reef Octopus 3000 Internal on my 180 Gallon System and I love this skimmer. The bubble blaster pump is amazing, and it pulles an insane amount of skimate out of system. I have used a couple of other skimmers in the past, and you just cannot go wrong with the Super Reef Octopus.
As for the Internal/ External question, there are pluses and minuses to both.
Internal
- Positive: Works great, if it overflows who cares? - its in the sump
- Negative: If you do not use an auto top off, it is harder to dial in
External
- Positive: Again, works great - water level in the sump is not a big deal
- Negative: If for some reason it does overflow, then you have a flood