It will depend on what you consider too slow.
If for you 8 to 12x it is not too slow you will be ok with an external drive.
Some will even offer you both firewire and USB 2.
Advantage of firewire comes from a simple fact - USB devices share the controller bandwith, so your 480 Mbit/sec will be shared by other devices and if it happens taht you have a USB 1 connected to the same controller (keep in mind that you can have several connector hanging from the same controller - you have to check what applies to your particular case) it will slow down the all thing.
Firewire works in a different way and that’s the reason why sometimes the firewire’s 400 Mbit/sec are faster than the 480 of USB. You have also to take into account the chipset.
Another point is that firewire is a 2 way connection and keeps a constant stream of data flowing through the channel, while USB depends on the machine Input/Output and only buffer gives the ilusion that there are no interruptions. This can be of interest for direct video burning - of the type of MiniDV avi files.
USB 2 is used for Mpeg2 video transfer, so it can be an option if you only require it.
Real disavantage of external - you have another box to carry along.
Plextor has a new drive that is rather small - the PX-608CU - besides it only does 8x, it can use all formats, including DVD-RAM.
Internal drives - take care because you can face some compatibility problems, dpending how your laptop IDE channels are configure and the drive comes from factory - master or slave.
Not every board or drive comes the same way, and when it comes to drives it is not as easy as with desktop internals that you change a switch to move master to slave.
Laptop drives may require a special firmware to do it, and some manufacturers don’t provide it.