We have a design in which we have a claw that grips onto a cap, and we were wondering what would grip more, the rubber bands that vex sells or anti slip mats?
NightsRosario is absolutely right, and it's also going to depend on your rubber bands and anti slip.
I buy alliance sterling bands from there and they have a ton of grip. If you're using vex's bands, you'll get a lot less grip. On the flip side, I get antislip from the hardware store (just for cost reasons) but vex's antislip has a lot more grip. Antislip also tends to lose grip much faster than bands, and both lose grip if they acquire a lot of dust. If you want to test whatever kind of bands and antislip you have right now, just slide them across the table.
If you are going with a claw, antislip works the best in my opinion. However, there is an easier way to make caps stay in if you don't use a claw. While you can also still use antislip, if you have zip ties on the forks and cut them the right lengths, they can hold in a cap fairly well from my experience.