Whacking or cervical dislocation is frightening to the animal even when done correctly because you are either restraining it tightly and/or whipping it through the air. Done incorrectly, it would be incredibly painful and terrifying.

CO2 (done correctly) knocks them out quickly and without them even realizing something is happening. I've even seen it done (by priming the chamber beforehand) where they are unconscious within a second or two. It's also a lot harder to do wrong.

The other thing about whacking is that it can get very messy. I'm no longer going to be buying from a local breeder I just recently got some rats from because there is always a bloody mess when I thaw, and he told me that's how he "euthanizes" (I put quotes around it because euthanize means PAINLESS death). It's really gross rinsing them off and it makes me not want to hold my snakes without giving them a bath first because any blood left is surely rubbing off on them! With CO2, you MIGHT have a slight amount come from the nose, but I found that if you put them in the fridge for a few hours first, then freeze them, you don't get the bloody nose.