For me it comes down to three things: style, fucntion, and price. The externals do burn at a higher rate 16x for the newest ones, but if you do not have highspeed firewire or usb 2.0 on your sytem then transfer rates can be really slow eclipsing the faster burning time. Needless to say an external burner is not as functional as an internal burner nor nearly as stylish.
Ultimatley, for me it was a matter of cost, for a hundred bucks more I got the best of both world, great preformance from a mac burner albeit at 8x and the ease of integrated hardware. Mmm..the only draw back is with an external burner you can always chuck it and buy a new one when even faster hardware comes out. For not 8x is fine for my needs.