I have 4wfs on a fluid element and thruster on  a riot boogie.   Both systems give you a wide range of fin sizes.  Thruster boxes give you more front to back adjustability in the track.  Fluid element provides some of this by giving you 2 4wfs finboxes per side, so you can pick the front or back mounting as you like.  4wfs allows you to change the fin angle and remove fins very fast.  This is nice as I run the parallel "speed" setting on the element rather than the inward splay fixed in the boogie.  I do also pop fins on and off per session.  Boogie seems to "hunt" because of the inward fin angles with the only option being to run smaller side fins.  4wfs is harder to find in the local shop, although they are quite responsive over the net.