Personally I like external better - its just more versatile since I have a number of computers. That way I'm not constrained to use a single computer to get some files off of my cards. Plus its portable so I can carry around if need be.
I wouldn't worry too much about xD cards unless you have them already. xD IMO, is really overpriced and it hasn't had much market success. Its clearly overshadowed by compactflash, and now SD. I think it'll die eventually.
The newer phone models are using miniSD cards, but I'm not sure if there are card readers out there for those. (must be somewhere, I just haven't seen them)
I heard of another format coming too, but the name has currently slipped my mind. microSD i think...