They are all pretty much the same these days eh. Some of the cheaper ones slow down more when they get full but not by a great deal.
www.tomshardware.com is a good place for benchmarks
something to consider is warranty. it use to be 3 years was standard, not any more.
if you're buying from Ascent.co.nz then you can pay a bit more for one with a 3 year warranty. At least they do that with the Seagate drives.
I personally use Maxtor cos I've had good luck with them in the past but some people havent and choose to stay clear.