Not what you asked- but unless you're dispatching from a cage- a rifle is almost always better.
If you are humanely dispatching from point blank range- just about any 5-6ft lbs air pistol will do.
As to your question- I believe the HW44 is one of the best spring air pistols out there. They command strong money for sure. But unless it has to be spring powered- a PCP pistol (if it has to be a pistol)- may be easier to shoot accurately.....