Hi Graf
We've got a quiver of each, they are both quite amazing kites and we far prefer them to anything else we've tried. They will both boost/jump high, turn quick and are great for wave riding.
I'd choose the C4 for riding a twin tip or strapped surfboard, big jumps, quick smooth kite loops, powered moves or unhooked riding. It tends to float you across the water with a smooth feel and plenty if punch for tricks.
I'd choose the Catalyst for unstrapped wave riding, smooth controlled jumps, exceptional depower or as an outstanding allrounder. It tends to allow your board to follow the water surface with a smooth responsive feel. If you're new to kitesurfing this would be the better choice.
Hope this helps

Nath