imho, i think that you should go for the internal.. i have a tower with both, and it seems like the internal drive goes a whole lot faster than the extrernal drive..
it really depends.. if your doing intense stuff with big files that require fast rates, id go for the internal drive. however, if you wanna use the drive with more than one computer, which can be a big advantage for swappin files, then id go for the external.
the thing is, also, that to some extent it doesnt matter. like ppl will recommend external drives for programs like fcp (and video editing in general) although they are slower because fcp doesnt output data faster than the max speed of the external drive. same with apps like carracho.. the highest speeds most servers have are around 150k, which is a ton faster than the max output speed of an external drive.
so basically, i guess, eitehr will work for you. one other advantage of an internal is the OS, for instance the os on the internal drive (10.1) loads faster than my 10.2 OS on my external drive...
good luck!