In terms of electrical conductivity, they're all pretty much the same. But start picking up the switches and activating the switches. You'll note that some of them trigger off fairly easily, while others require just a little bit of extra "oomph".
It's been my experience that Cherry switches are the "easy" ones, while Zippy require more force to trigger.
It all comes down to personal preference. I bought some buttons from Competitive Products for my Project Blue cabinet, and they came with Zippy switches. They're too tight for my tastes, and one day I'm going to trade them for some nice easy Cherry switches.