Actually, a 1TB HDD would be much faster overall. The bits are more densly packed in than 500GB HDD's (depending on model/platters of course).
Also, as hard drives fill up they tend to get slower (around the 50% filled mark?) - so a 1TB HDD would be "faster" in that regard.
But it all depends on your needs. Do you download/save a lot of stuff? If you answer no, then you can get a 500GB HDD and save yourself some money.