I've done both - recorded to external HD and internal. In theory and for most people, in practice, recording to your boot disk is not as efficient.
If you MBP has a 5400 rpm drive then getting a FW drive with 7200 rpm would be better for recording to or streaming samples from.
However, you know that MBP's can be bought with a 7200rpm drive. That's what I use, and it works well.
If you're only recording one track at a time then you may not notice the difference. If you are streaming large sample libraries (e.g. Omnisphere) then you may be very happy with a 7200rpm ext. drive.