The trade-off you're probably going to have to consider is capacity vs portability. You can get large capacity external drives for a reasonable price, but you're going to get stuck with a separate power supply to drag around with you. OTOH, for a comparable price, you can find smaller (in size and capacity) drives for a similar price that require no extra power supply.

For my part, I have one of each. A large capacity(250GB) drive that I mainly use for backups at home and a smaller(100GB) USB-powered one that I keep work-related files on to take with me to sites.