The G403 has a significantly better sensor than the EC2-A, but is that human noticeable? Probably not.
Things you would notice though.
G403 = ~87g (can be easily modded down to about 82g if you like ultra light mice)
EC2-A = ~93g
Click latency Zowie mice are about ~7ms slower than logitech mice in general
Finally something I have no idea how to confirm but according to this
http://recklessyuki.com/mouse-database/
Zowie mice are 2ms faster at sending tracking information on the X axis (left - right) and 8ms slower at sending tracking information on the Y axis (Forward - Back) (technically Z axis if you ask me but whatever)
you can see some of his testing methodology here
https://www.youtube.com/watch?v=wO22BuqLnYc
Keep in mind the margin of difference between these 2 mice isn't that significant, whatever is closer to the shape you prefer and you like the feel of most is probably going to be the better choice for you.