IIRC, you get three tunes. You can make the three tunes do any three things you want, but usually...
One tune is for regular, 87 octane fuel. Mild performance increase, very mild MPG gain (if any).
One tune for towing, with 92 octane fuel.
One tune for performance, with 92 octane fuel. Awesome performance gains, and mild to mediocre MPG gains. For the most part, you probably won't notice much of MPG gains because you won't be able to keep your foot out of it.
The cool thing about the SCT is that it can be customized... you can have Hemifever do tunes for 91 octane instead of 92, you can have him not do a towing tune at all, if something doesn't feel quite right you can contact him and he'll adjust the tune and get back to you, etc. etc.
The SCT is definitely the way to go. It is on my short list now, as soon as I get around to getting my Superchips 3815 sold (will be up for sale relatively soon).