Well that depends on the drive's specs, but in most cases I'd say that the internal HDDs beat the external drives, especially if you are talking about a portable external drive. I know that you're asking about performance, but generally the internal HDDs are a lot more reliable than external ones, which would considerably turn the scales in their favor if you're wondering what to get.
As for the performance, if you give me a bit more info or if you're looking at specific drives, I'd be able to give you a more definite answer.
Hope that helps.
Boogieman_WD