RE: R18S vs R19S vs R13M
With SB 160 my best fin was R18S-(+8). I changed R13S+8 because the R18 had more speed at same upwind angle and more easy to go downwind. The R18 is more technical than R13, but if you can handle it, the R18 shows good performance (8-20 knots).
To find the right fin it???s also the matter of personal preference, rider???s weight, style and technique.