Both pedals have a use in my opinion but I am biased and use many different pedals.

I would set them to differnt levels of boost so one increases volume and the other pushes the amp even harder into break up. You can also experiment with both pedals on together for another tone.

Personally I like using the TS9 with the volume up fairly high to nearly full and then use the gain control to set the overall volume of the pedal, I think it works better that way than using it like you would a conventional pedal.