My guess is Dennon is only going to support SDHC out the gate as there is no SDXC logo on any of the information anywhere. The good news is it is something they could licenses can patch in later down the life of the product.
As for what you should use, in theory since both the SD and USB sticks don’t have any moving parts (like a HDD) they both should have great reliability. With that being said, USB sticks have much better shielding and protective casing so I would stick with that as well. Also providing that the SC5000 are supporting USB 3.0, the USB option will be the fastest option providing you find a stick that can handle the IOPS you throw at it.