I actually made a pilgrimage to Mecca (the Sweetwater store in Fort Wayne, IN) to check out the Prophet XL and others.
For anyone who needs it:
All these keyboards have the same octave length (16.5 cm C to C):
Prophet X
Prophet XL
OB-6
Prophet 5
The Prophet X keyboard seems to me to be exactly the same as the Komplete Kontrol S61. The nice Fatar synth feel is there in full force.
The Prophet XL keyboard is different though. Not only are its keys differently shaped (see pictures), but the pressure required to press a key is much greater. Due to this, it can't really be described as "synth action", but its upward snap-back is such so that you can do fast synth-style runs or "manual arps" much easier than on a hammer-action piano keybed. And since it requires more force, it doesn't feel weird playing piano parts on it. It's effectively a hybrid keyboard feel - which totally suits the hybrid nature of this instrument. I really like it actually... I'm more used to piano action anyway and the extra snap back is much appreciated when playing fast parts. I played it a long time at the showroom before I decided to take it home with me.
One thing about the XL too - the aftertouch is tricky to work with. Since you have to press harder to press the key, it's really easy to accidentally trigger the aftertouch. So you have to hit the key with just the right force, or you may get an unintentional vibrato with patches that have that. I'm still getting used to that.
Now if I could just get this thing to record MIDI over USB, I'll be set; but that's another thread...