External drives can have trouble burning above 8X if you don't have a Prolific 3507 chipset. Otherwise they work for most aplication with no problem
I would avoid the combo drive as they have more problems and few have a wide range of firmwares available. Also, they do not have burn quality as good as dedicated drives. You don't need two drives if you choose a DVD burner that does both well.
Firewire and USB have the same limitations when it comes to burning. It looks like USB2 would be faster (480 vs 400) but this is not the case. If you get a case made by Plumax, they have the correct chip and make a model that has both connections.
The NEC 3520 is my first choice for currently available drives. It has the best burn quality for my choice of DVD media and is a pretty fair CD burner.