Zowie EC1-B has smaller mouse feet, the S1 has big mouse feet like the EC1/2-A does.

I was actually using the EC1-A before my S1 arrived, the S1 is slightly smaller, and in my opinion better, this is coming from someone who used the G403, IME3 (Intellimouse Pro too) and EC1, Rival, Deathadder etc for years each and a Air58 (2 weeks lol).

The S1 is like a size between EC1 and EC2, right in the middle, some people find the EC2 too small, the EC1 too big, this is the middle ground. I assume you palm/claw or hybrid grip these and this is perfect for the medium large to large size hands.

I palm grip it and find it comfortable, honestly the right hand grip mice main advantage is the way the hump is ergonomic, an ambi mouse that allows this (such as Finalmouse and the Zowie S ar just as comfortable) so I'd recommend the S1. But if you have a giants hands get the EC1.