typically a quad will be faster from the take off...more instantaneous speed, while a thruster tends to need to be "worked" a little more to get up to speed. quads tend to be "looser" than thrusters as well...depending on the shape (i'm not familiar w/ doc's shapes at all), the quad will really play into the "skatiness" factor of a board more than a tri fin would.
personally, i've found that quads take a bit more experimentation & patience to get really dialed in w/ the right fin set up, while thrusters are more "plug & play" for me. don't get me wrong, i've really been liking quads lately & have started getting the majority of my boards made w/ the tri/quad option, but i find that it takes more time & effort to really figure a quad out than it does a tri fin. but i still like thrusters, too.
I like quads in smaller, weaker surf as they help generate speed better than thrusters. As the waves get bigger or more powerful, the emphasis shifts from generating speed to maintaining control at speed... at least for me... because the wave provides all the speed I need.
I have the K2.1 set and love it. I haven't really tried any other sets on this board but hey, its Kelly Slater. He doesn't just put his stamp of approval anything old thing that comes along. I'm actually trying to get that little 5th fin that he uses on bigger surf with the quad set up just for kicks to see how it works.
yeah, I might get those... actually, probably getting those. Also thinking about getting that smaller 5th fin. A friend of mine just got it, but hadn't have a chance to test it just yet... He expects it to allow him to surf on his quad set even when it's bigger and hollower... I may give it a try too.