SDXC on MBP 13" and MBP 15"

My boyfriend and I have both MBPs (I've got the 13" model and he's got the 15" one). Since we have both the SSD 128 GB installed, we were wondering if putting an SDXC card into the apposite slot could be ok to increase some memory. The original idea was to put iTunes stuff (like iPhone apps and music etc..) into it so to save some space on the SSD.

The thing is: would it be slow? Because we have both a portable HD with Firewire800 connection but using it would not be the smartest choice since we should have to carry it along with us all the time.