Cost mostly (at least that's what I found when I was researching). I wanted something in the 1200-1300 gph range for my 120. I found that most in sumps at that gph are very expensive. So I opted for a pan world pump.
Second is maintenance in the fact that the pump gets dirtier quicker which can affect your gph quicker than with an external. Yes, all pumps need maintenance, but all my previous in sumps needed more tlc than my current external. (That could be a result of having higher gph, but not definitive either way) I got tried of needing sponges to protect the in sump impellers, and dealing with all the crap that may not be caught via the filter sock, or slime build up. I also "think" you get more gph per buck in the world of external pumps.
I switched to external while my tank was still running fyi. It's plausible and not difficult if you're setup correctly plumbing wise. I cut the flow to sump, cleaned out the sump thoroughly since I had it dried up and everything. Dropped at lfs to be drilled, and picked up two days later. viola piece of cake.
You can certainly do in sump and I'm not bashing them, they have their pros, I'm just glad I opted for external. And got more gph than I thought I would need.
