Sounds like you have most of what you need. You didn't mention covers for the tanks, which are a must. Your hide ideas sound fine and you're right about needing a water bowl. I wouldn't worry about a humidity gauge. In my opinion they aren't necessary, and with a glass aquarium and an open top (screen cover of some sort) you will be hard pressed to keep the humidity at anything other than the humidity of the room in which you keep the tank. As long as you mist when they are shedding you should be good to go. Lastly, I wouldn't worry about PAM unless you actually end up with mites. I feel like it would be bad luck to buy it preemptively.

As for which snake to get, pick whichever one you like the most. As long as they are both heathy it should come down to your personal preference.