Hueco, I work at a boys camp in the summers over here in California, and the whole take the snake with you thing is BS. Snake venom is pretty much all the same, so they don't need it. Plus, the majority of bites are actually dry bites, meaning they only bite you and do not inject you with venom. IF you should be bitten, the best thing to have a one of those yellow Sawyer Extractor hickie givers. You do not cut the bite, you do not use a turnaquete, you simply stick the Sawyer Extractor on it, push in the plunger, adn let it suck while keeping your bitten extremity below your heart and get to a hospital as quickly as possible. Just keep in mind more people die of bee stings a year and the calmer you are, the better off you are.
The only thing that is going to happen when you haul in your headless rattlesnake or blown in half rattlesnake in with your bitten butt is probably laughs as they chuck it in the dumpster.
As you said, snakes aren't a problem, you just walk around them. I used to have that macho kill 'em all attitude, but I quickly learned to like running into rattlesnakes on the trail. It is a good exhilerating experience and they are usually pretty docile and interesting to observe.
So despite my feeling there really is no need to kill a rattlesnake as they are pretty harmless if given their proper respect and clearence, I would say some dove or qual loads of 7 1/2s or 8s would be seem to be the best load for me. I remember shooting the head clean off a rattlesnake with my little 20 gauge at about 10 yars when I was a bit younger. I think that the smaller but more abundant shot would do just the trick. Besides, with rattlesnakes, you are probably not going to be shooting at more than 10 yards away. One they coil up you can get with 1-2 yards and they just stare at you with a pissed off look.
My main point here is, leave the snakes alone. They eat vermin such as squirrels, mice, and rats. They are really docile creatures and 99% of the time they only want to get as far away from you as possible.