A dead one is better, since occasionally the prey becomes the predator. I only feed my BP live, when I got it it hasn't ate since the person i got it from got (which was 6 months) and it took me 7 1/2 months to eat. I finally got it to eat live mice I hold the mice by the tail and let the snake take it from my hand(really stupid i found out with a 12 foot Burmese Python). If you can get it to eat dead it is the ideal way to feed them.
Live mice can bite the spine and paralyze(spelling) the snake or just chew thew crap out of the snake.
I only feed my snakes dead mice unless the pet shop has actual live pinkies. Baby mice don't have teeth, so it isn't harmful to feed live. When my corns graduate to larger prey, I will definately stick with dead mice. I'd rather have a few dead mice in the freezer than a few dead corn snakes. It may not be as cool to watch, but it is still food to them. They eat anything.
There was an article in the Vivarium a few years ago that suggested that previously frozen rodents were actually more nutritious than non-frozen because the freezing process breaks down the tissue and make it easier to digest.
The only reason to use live is if you have a snake that absolutly refuses to take frozen, but even then it's bennificial to kill the mouse immediately prior to offering it to the snake.
BTW, in case any total snake newbies are reading this, when we say "frozen" we mean stored frozen, but totally thawed out before feeding. If a snake eats a mouse that is actually frozen, it can kill the snake. This seems obvious, but I've heard of it happening!
Frozen thawed is the best choice hands down. No parasites and no risk to the snake. Not to mention a tremendous cost difference with frozen being far cheaper. Live mice are really only good when the snake in question refuses to touch frozen which most snakes will readily take. Too much risk involved with live mice the largest being that if a snake takes a bite from a mouse inside of it's mouth it can develop a bacterial infection and die from it. The smaller reason and still unacceptable to me is a bite on the snakes body which can cause permanent scarring and even worse lost eyes. Nothing quite like hearing a story about someones prized super nice looking boa suddenly losing an eye to a rat. Happens more often than you may think. I do have 2 snakes that eat live but that is simply because they wont take frozen one being a chondro the other being a diamondback. Not too much risk of the chondro being hurt though and no risk on the rattler.