ESL vs FEC vs COWOA

I'm not really sure what the difference is between these, and what would be able to work together with each other.

I'm looking for a "Checkout without account" and "Easier Checkout" type mod, is there one or two of these that work better together?

Would be great if somebody could explain.

None of them will work with another, as they all have to modify some of the same files; you would have to merge code between the mods, which would be tricky at best and probably a nightmare to reconcile the differences.
You will really have to try them and settle on one that works best for you. You can mod from there if you want.

I heard that FEC has COWA integrated.

An Ajax Checkout is on the way :), I have it completed 90%

I think I'll give FEC a shot... sounds like a more complete solution. Thanks guys.

cool, Ajax... Can't wait!!!

FEC is no doubt to be a great pluggin for Zen Cart but the small lacking which was faced by me was the Telephone Number for the Shipping Address. Our client needed this and it was a great time for to start exploring Zen Cart. We got the Shipping Telephone pluggin but as we were using FEC, it didn't work. So we built that section on the FEC.