MicroUSB has little built in springs to help hold it in, plus is designed so that wear and tear will more likely happen on the cable and not the socket (unlike MiniUSB).
And are you talking about using a female USB A socket on the keyboard? That'd severely limit your cable options.
Personally, I'd either go with a USB standard B (older, original, big socket still used on printers), or a Micro B. MicroB is kinda hard to solder though if you don't do a lot of soldering with small stuff.