my opinion:
for "normal" everyday use files, I prefer to keep these on my internal just for ease of use (I would get annoyed if i had to hook up my external all the time). for "stored" files like video editing files or backups, i use an external b/c I only have to hook it up once in a while.
also, ppl opt to upgrade their internal HD space b/c of the general trend that you'll probably end up needing/wanting more than you originally got.
if you don't mind always hooking up the external... thats fine. but it would annoy me if it was a daily task =/ but im lazy =P
I opted for a 500 GB HD and use my external only for time machine backup