Actually, it really isn't going to matter much at all what type you buy as long as it is the appropriate physical dimensions for your enclosure. Both will easily be able to make a 90°F hot spot for you.
What is going to matter, regardless of which one you choose, is connecting it to a quality thermostat to ensure those temps don't get any hotter than is safe for your new addition. An unregulated heat mat is not something that you want to subject your ball python to.
Lastly, substrate choice is going to be up to you. The most important factors are going to be how easy it is for you to keep clean, that it is not too thick to get in the way of your heat pad working properly, and how well it holds humidity (only if you have issues with that). I use Repti-Zorb liners in all of my tubs and don't plan on changing things up any time soon.