I went with the 507 since it came with 4 sensors. I needed 6 total so it saved money for me to start out with 4 because the sensors are $100 each. I also got the flow through sensors which I would give a pass to because they are so restrictive, takes forever to add anything over a couple psi.
If you have a long traler, 38' or so, you might also want to get the repeater. Assuming you're buying for your trailer and not just the truck.