Speedplays all the way.
If you are not totally satisfied with the Keo, than why not try a pedal that operates on a completely different principle. The Shimano, Campy and Look pedals all work on the same cleat/spring retention principle, and thus share most of the same engagement/disengagement pros/cons...the Speedplays work by utilizing a spring clip design and the retention mechanism is now part of the cleat and not the pedal (so now you have an entirely new set of pros/cons..hahah) Once you get used to them they are super fast in and out, they are very lite and easy on your knees...and as far as cleats getting clogged, that's really more of an issue if you are using them for MTB riding - In which case you should go Crank Bros or Time.
The single-most common issue I have seen folks have with the Speedplays has to do with stability issues in the ankle, this athlete may not like the "feel" of the X series, since its retention does not "self-center" the riders foot over the pedal. In the recovery part of the pedal stroke the weak ankle will allow the heel to wander side to side, thus giving a feel instability (ice skating is the analogy usually given) which is readily visible and sometimes can lead to a premature release. The Zero series manage this with there cleat design which limits or can completely remove the amount of lateral rotation possible before disengagement.
Speedplays are light, minimal, easy on the knees and being double-sided, easy to engage.
See if a shop will let you try them out...