Go with the PSI that is specific for the car. The tire manufacturer didn't know what kind of car every tire was going on. It's also listed on the back of the glovebox door.
50 psi is for best wear, 35 psi is for ride comfort. This is becoming a HUGE issue in the Tire Pressure Monitoring Systems that are on cars now -- you go to replace the factory tires and you can't get the cheapos that are rated at higher PSI unless you want to replace the tires again in a few years. But like I said, 50 for best wear, 35 for ride comfort.
Awhile ago, I was talking to a mechanic, who said to fill the tires up to about 5PSI less than max PSI for better tread wear. SO far the tires have been on the car for six years now and look like they'll last another six.
If you read the markings on the tire you will notice that the max 50 psi is also at the maximum wheight rating, ie: 2200 lbs. Do the math, with 4 tires at max weight you could support 8800lbs, and their is no way you will ever have 8800 lbs on the tires while driving a Mustang.
the rating by the auto manufacturer is going to be the closest guide and you could adjust from there to find your best balance between ride quality and wear.
but bear in mind the manufacturer rating was speciific based on the tire size that the car was originally equipped with
Gord