I realize the notion that we can understand the level of pain being induced is subjective, as no BP will ever be able to convey the experience, thus we guess. That being said, there are a few pressumptions that I tend to side with, one being that freezing a reptile would be slow and agonizing as they are cold blooded and the metabolism for such animals, works in the way it does.

Aside from that, lets assume that the neurological pain sensory systems are similar. I would think a needle puncturing the heart would be extremely uncomfortable.

Further, while I agree that lower metabolic animals take a longer time to quit showing involuntary movement, I would be surprised to discover there would be any real pain associated with decapitation on either of the two segments. My logic though unprovable, is simply because the trauma would cause shock, which at least in mammals, manifests in the form of numbness. Just my $0.02 which is as worthless as anyone's opinion on what a BP feels.