Here's a nice graph of the lumens per watt vs. current for the Cree and P4, though as ginaz said the output can vary from LED to LED even within the same bin.
https://www.candlepowerforums.com/threads/152094
The P4 is MUCH easier to solder and for the most part works with reflectors, heatsinks, and optics designed for the Luxeon I, III, V LEDs as it is almost identical in shape (round) and size, except the P4 sits a little bit lower than the Luxeon. The bad things about the P4 would be the fact that the bottom is electrically connected to the anode (+ termination) so you have to be sure to electrically isolate it from your heatsink, and the P4 also has a gummy / sticky dome that is a dust magnet. It's a little hazy too, not nearly as clear as the dome on a Cree or Luxeon.
The Cree is a completely different footprint (rectagular instead of round), but, the Cree has a nice dome and the bottom of the Cree is electrically isolated. The Cree, in general, isn't as bright as the SSC P4, but it is very well packaged. Problem is, it dissipates heat so well that if you use AS or AS epoxy to connect it to your heatsink BEFORE you solder the wires to it...well, when you do try to solder you'll find it to be almost impossible as the Cree will suck the heat away from the solder pad and into the heatsink and you'll never get the solder to melt. One more thing; the + and - solder connectors on the Cree are on both top and bottom, so you have to either clip the corners off of the Cree (where the traces run from top to bottom), or electrically isolate the entire LED. I prefer clipping the corners...
It seems that Philips Luxeon is back in the game now too with their GenX LED- you guys seen this thread?
https://www.candlepowerforums.com/threads/150005