I all depend on what you need. Do you need lot of space for personal files which you always wanna have with you and your laptop, or do you have lots of movies, ISOs that you use on several devices (laptop, dvd player, other computers..).
For the first need I recommend an internal drive, while for the second demand it would be best to have an external HDD (which also needs a power connection, don't forget that, because it's not that easy to always carry an external unit with you and your laptop)
If you want a good internal drive, also very good for booting purposes, I'd recommend the Seagate Momentus XT 500GB
Hybrid drive which combines a 500GB HDD with a 4GB SDD memory