It depends entirely on how much you weigh, what tyres/rims you run, what terrain you ride and simply what you prefer. I run different pressures for street and certain types of natural even though my weight/rim/tyre is the same. Try going out one day with a pump and both tyres at, say, 40psi. Try letting each tyre down by 5 or 10 psi at a time to see how it affects you. I run about 22 front and 18 rear on street, a little more for some natural.