Which chomper for pvp?

As mentioned, Hot Rod's speed boost can get you in and out of trouble very nicely if you're careful with your chomps.

Electric Chomper is an old favorite of mine, because he does a pretty good job of doing good damage to groups - goop the capture point, and pop out of burrow to do some lightning breath. You might not be a kill machine, but it will at least help other plants take the objective.

Count Chompula's health on swallow could be equally useful as Hot Rod's speed, helping you survive and re-engage after a chomp.

If you want to do some range support, Yeti Chomper is the clear winner. Good ability to do damage as you approach, and then chomp.