A few things. First of all, the only difference between an internal and external HDD is the external chassis; the HDD inside is the same exact type found in your average PC. Secondy, the brand of the HDD itself is much more important than the brand of the chassis it's mounted in.
For HDDs, I like Seagate and Western Digital. I recommend you avoid Maxtor and IBM HDDs. If you're feeling brave, you can actually buy the HDD and the external case seperately and build the thing yourself. If you were to do that, you could save a lot of money and ensure that you get exactly the HDD you're after.
All the best,
-AT