Which hard drive would be better?

The most expensive one! Sucks though on price per GB!
10.000rpm will be faster.

If money isn't an issue, then why bother with an HDD?

Get an SSD:

http://www.newegg.com/Product/ProductList.aspx?Submit=E...

For regular use just got any size HD in size you want 7200k RPM are faster.
Depending on enclosure you use, 5400RPM drive will be better. (less heat so it won't fry)
SATA II is same as (sata, sata 3, eSata) it will work inside enclosure.

And forget about Enterprise 10k HDD, its waste it meant to be on 24/7 (and other server features) for that $$$ just get SSD Drive. Thou you have to make up your mind for what use exactly your want it.

If money is no issue, buy a SSD in the capacity you need. The SSD is about 50x faster in random i/o and 2-3x faster in sequential. Expect to pay $1-$2 pergb.

Look to Intel or samsung first for reliability.

The momentus XT will be better than any other conventional hard drive, but nowhere near the performance of any ssd. They now make a 750gb unit with more nand cache that should perform even faster.

Enterprise drives, even 15k rpm drives will not perform well in a desktop environment. I know, I tried them some time back. Their microcode is optimized for server work. They were nothing special.

The velociraptor was a good drive for the os in it's day, but the ssd is just so much better.
You will not regret buying a SSD.