I've used both types for years. Never had one fail yet. It kinda stands to reason though that one you plug/unplug all the time and transport around would be more likely to develop problems.
Thinking of speed ... the specs are usually given with the drive ... fair to say that internal are in general quicker than external but how important that is depends somewhat on what you're using it for ... I mean how fast does it have to be to watch a blu-ray movie ... similarly, if I'm copying a movie ... i always use low speed (2X) so for me at least ... a 12X burner isn't much of a selling feature. I think you'll find the media is more important than the burner.