I thought the RPi only supports USB 2.0?
The USB interface of the RPi is USB 2.0, but all USB3 sticks are backwards compatible to the USB 2.0 interface.
And good USB3 sticks are much faster than USB2 sticks, even when used with a USB 2.0 interface.
USB2 sticks never even get close to the max capacity of such a USB 2.0 interface, but USB3 sticks can exceed it.
So with a good USB3 stick you will be using that interface at or near max capacity.
I'm currently shopping for storage to put in a RPi for Rasplex.
With the latest improvements to RasPlex 0.4.0-rc4 there is only a slight GUI speed gain to be had from using USB3 sticks, as compared to using the best SDHC cards with UHS-1 certification. And if you use such a card without a USB stick the installation procedure is much simpler. (No need for special tools to expand Ext4 partition, as you would need if using a USB stick.) I've stopped using USB3 for my own RasPlex setup, though I still use them for some compatibility tests, and with other non-RasPlex programs (mainly BerryBoot).
So in your place I would go for getting a top quality SDHC card. This means one having both the Class10 logo and the UHS-1 logo, and preferably an explicit high-speed guarantee for both reading and writing.
The Class10 logo looks like a capital C 'surrounding' the digits "10".
The UHS-1 logo looks like a capital U 'surrounding the digit "1"
Here is a scan I made of one of the cards I've used recently.

This shows the appearance of the two logos I mentioned, which should be similar on cards of other brands as well, even if their placement on those cards is different.
Just be careful to get SDHC cards, and not the newer SDXC cards, as those are most likely incompatible.
Best regards: dlanor