I think the fish vs regular thruster thing is not so much about low wind, but about riding style. Sure, you may get out a couple of knots earlier with a fish, but you will not be able to ride a wave properly down the line. A well designed surfboard that has been built with kiting in mind will get going in 12-15 knots. This is also about the minimum wind needed to ride down the line otherwise you will keep overtaking your kite. A fish with the right kite might get going in 10 knots, but really all you will be doing is going backwards and forwards. So, if you are looking to really surf waves with a kite, then the question becomes do you want a performance board or a fun board?
By the by, on the thruster vs quad thing, it is much more complicated than saying that a thruster turns better and a quad goes upwind better. I have ridden thrusters that had great up wind ability and ridden some quads were useless at going upwind. Board design, fins, fin angles, etc will all dramatically vary each board. Unfortunately to really get an idea what you like you need to try a bunch of each. Personally, I do feel that thrusters do allow a more vertical top to bottom approach to surfing that appeals to me. By the by, I regularly get out with a 6'2" surfboard and a 7.5m Generator kite in 12-15 knots.