I could give you a BS answer and think of some reason that I think live is better for my boy, or steal an answer from a more educated and experienced snake owner... BUT...
For me the honestly for real with facts answer is that I enjoy watching Jackson hunt and kill. Sorry if that offends...

When his prey gets bigger I will give him p/k, and he does eat it (I've given him f/t before, he takes it with no hesitation). Occasionally the pet store I go to is out of the rat size I give him so I get an adult mouse- and I kill it before I offer it to him. This is because we did have a bad experience with one giving him a good sized gash in the neck.