First of all I have to drop a sanity check on you: do you NEED to buy a dedicated server to host one low traffic website?!?! Don’t waste your money on dedicated hardware if you have no use for it, the world doesn’t need the additional future e-waste as it is.
Secondly to actually answer your question - all things being equal, if storage size doesn’t matter to you (lucky), then go for the faster storage. Just keep in mind that you will more than likely be running RAID 1 aka mirroring. So you will end up with only the capacity of 1 of the drives as your total available storage pool.
Good luck. (You don’t need a server for a single website, don’t be foolish)
100% - couldn’t agree with you more.