This comparison is meaningless as I don't believe I ever saw anyone make such a comparison. The comparison is between the Seagate Momentus XT (or like drives) and a modern day SSD. In such comparisons SSDs beat the Seagate Momentus XT hands down. Unless $/GB or the highest capacity in a 2.5" form factor is a compelling factor I don't know why one would opt for a spinning hard drive over a SATA SSD in a 2009 Mac Mini (or most other consumer based PCs). You have yet to make the case for it.
I think I did. I stayed on topic as the OP was only willing to upgrade if the hard drive he is replacing in the Mini is larger than stock, which was 320GB. I merely addressed what the OP had outlined and what other options can be deployed to see some real benefits, while some of you are suggesting different computers and smaller SSD sizes which are off topic I think to the Mini 2009. While the Momentus XT was slower by a few seconds compared to a true SSD and that is to be expected, it is not by much but you do get 4 to 6x more storage space than a SSD for the same price point. There is no need to run 2 drives (1 SSD and 1 spinning platter.
But then again, it is your money you can spend it on what you like.