Been using the G403 for a while now and I would highly recommend, especially if you have a larger hand like I do (21 cm x 11cm). Gripping as claw or palming it is pretty comfortable either way you go, and as someone who plays a lot of medic/CS, I really like the way the scroll button feels for bhopping if that's your kinda thing. The sensor has been praised by pretty much everyone and for good reason, it performs very well. Though it is a bit heavier than some other mice, I actually like the heavier feel as for doing things like flicking for xbow/rocket airshots it feels like I'm not going to overshoot a lot more compared to my old mouse which was about 15g lighter.
Couldn't give you a fair comparison with the EC1A/EC2A as I've never personally owned one, but the key differences people usually bring up seem to be a better sensor with a similar shape design and better M4/M5 buttons.