Do you sail at Sandy point?
21 cm tectonics seems awfully small. (I think Steve thorp was using a 21 or 22cm tectonics at Driven by wind and found it too small)
I've used Isonic 50 with 24cm Tectonics and It was great, currently running the 24cm Tectonics on the SR50, and again just great especially downwind.
It seems Mals fins are going well, Bob Cunnigham and Farrel'o'shea are using to great effect here in the UK, and of course all the sandy point crew
I'd go for 24cm tectonics or one of Mals asy fins