I have a seagate 2TB desktop type that cost £69 and is good and fast. I personally feel that the desktop ones are a less likely to fail as they are static whereas the portable ones are bashed around a bit more. I don't know if the actual drives themselves are any different, I doubt it.
As above, I will eventually get another one to backup the backup, but one is better than none for now, and really important files are in the cloud too.