Hi,
It depends on how far you want to go, or intend to go, in the future.
With OpenCockpits it's modular, so if you just want to stay with a few switches then USBKeys may be your answer, if on the otherhand you intend to gradually build a full sim then it might be worth going for the USBExpansion/Mastercard option, and slowly add to that.
Have a read of what each card does, to get an idea