I have the Gap Diagnostics tool non bluetooth version and has worked for me, I have never used a faultmate but it looks like it can do what the GAP tool can do but in a fancier presentation. My GAP tool stays plugged in all the time, and displays voltage on the information center, and it can also diagnose suspension faults. On my rig I use it to turn off suspension when I get a fault (most common for me is "frequent height adjustment" fault), and worst case scenario I have an emergency air up system to individualy inflate each air strut.
I would go with GAP tool, it was delivered quickly, but when you first get it you will have to register it online and enter a code on vehicle.