well, larger wastegates take space and cost money. if you can do your job with a smaller one, do it. generally, you need a big wastegate when you wanna run low boost on a big cc engine. the more boost you run, the less open the wastegate is, so...
the really good thing about an external (when its got a screamer pipe) besides the sound, is that it effectively makes your exhaust bigger
...just by a bit.