To get things working, just install the free driver and pop in your card of choice. You'll be able to use any USB 3.0 compatible device with your Mac at will. Of course, your existing USB ports will continue to run at their default speed. These aren't magic cards, after all.
The PCIe card will run you US$49.99 while the ExpressCard costs $59.99. But hey, if you've got $2,499 for a base Mac Pro, fifty bucks won't break the bank.