I'd go internal as a "permanent" or full time one, although a backup external would be handy to have, as CD/DVD Roms can be finicky creatures and crap out suddenly for no known reason it seems, lol. Internal would be faster than USB, plus use the internal power supply - an external would most likely need to be externally plugged in for power... and there aren't always enough plugs around.
I had one computer went through 3 of them in 2 months - obviously faulty - but under warranty. Finally got one that is still in the machine today and working - and that computer is... uhmm..... uh....... YEARS old!
I also keep an external harddrive to back everything up on. Slayton keeps stealing it for the music files on it. I lost 11,000 songs off one comp when the harddrive died - cried for weeks.
That taught me. All music cds/dvds that come into the house get ripped onto a computer, stored on a disk as a data file, and backed up to the external harddrive. And for my own ones, then play copies are made, the original goes on the shelf and is never used again most of the time. I should sell 'em.