No difference in performance - for a keyboard even much much weaker chips are sufficient.
The difference between teensy 2.0 (has atmega32u4) and pro trinket (has atmega328p) is that the former has hardware support for USB, whereas the latter only bit-banging in firmware. There is one library (vusb) that you have to use with the latter setup, whereas there's a couple better (=more friendly and versatile) libraries for the 32u4.
Also atmega32u4 is used much more often in these circles, so some firmwares that can be used are already around (e.g. TMK).
Anyway - between those two I would go for teensy. If you're a programmer skilled with AVR chips, and really want to save a couple of dollars, you can do slightly cheaper (e.g. with trinket (not pro) or a chinese knockoff of these; or a chinese knockoff of arduino pro micro) - but IMO it's not worth the trouble.
EDIT: just go with bpiphany's PCB.