I have seen an interesting feedback on this in Tyler Callaway's interview at twitter.com/erik_antonson

Personally I love the inside concave, the fins feel much more smooth in the water. But it seems a lot of people are hooked on the slight drag of flat-sided fins that provide them more control. Tyler says that the difference in the leading edge gave the feeling that the pro loved.

Some times ago, I did catch from JP argued by Piros that no cant, no toe and flat was working good...that's true and give the reason why:
Because as SUP board is very wide compared of surfboard, the less pressure of the area of fin allow to bury deeper the rail .
I mean that less surface in pressure on water more control you get on the board. In opposite to the foil fin settled with cant and toe which are more positive and therefore need much weight to get control on board and fins.

Johndesu said..
Thanks guys - I will also have a look at Tyler interview when I get time,

You can skip the first third, that's just trivialities.

Note that after seeing this interview I bought two Gerry Lopez FCSII rear quads, as they are small (nearly trailers) and symmetrical (50/50 foil), which is interesting for rear quads to smooth the control and give some thruster feeling to a quad setup.