Single concave, particularly in wider, flatter boards, feels kinday sticky to me (I'm thinking Rocket style boards). They try and make up for this stickiness by putting kick in the tail which for me just makes it stall. So personally prefer a moderate single not too far forward and some double between fins to vee out the back combined with a bit of entry rocker and low tail rocker. Seems like all the boards that I like, regardless of design or wave size follow that general template.