the 37 will jibe easiers as it is a freeride. This means it has a difrerent, mroe curved profile with less water resistance, however the speed will be lower then with the 40 freeslalom though considering both profiles i'm not sure how much difference there will be. Also the sailrange on the 40 will be bigger then on the 37 freeslalom. Personally i prefer sailing slalom fins because you can jibe at almsot the same angle by putting more effort into it and you have a higher speed. However, when the winds blow strong i prefer a freeride or crossover fin, they give you more controle over the board without losing too much speed on the way. Slaloms will be faster none the less but because of the speed the board will be harder to controle and you end up in very messy situations.
hope this helps
Haiko, AKA crazychemical