I have tried the firefly, butterfly and airturn ped pro. And I chose the firefly.
The reason is that it is the most firm and worked well when I am barefoot. clicking felt slightly more comfortable than with the butterfly, because I had to lift my toes a little less. The air turn pro is even flatter (so even more comfortable) but needed to much pressure when barefoot.
The firefly has the most options (usb, small light on the pedal, more choice for which setting) although this is not important to me. the only reason I chose the firefly was the sense of using it.