Hard to answer because in the real world, most of the time you have a 500K pot, its on a pickup that's higher in output and that has a different inherent e.q. than the pickups that usually come with 250K pots. So you can't just pick up a Strat and then a Les Paul and then make a meaningful judgment of what 250K and 500K respond like across the board.
Theoretically, with the same exact taper, and all other things being equal, a 500K pot should clean up more noticeably than a 250K, because at 10, the 500K will be causing the pickup to output a slightly fuller and hotter tone. Therefore your starting point at 10 is brighter and hotter than with the 250K. When you have more to lose, you notice the loss more.
That being said, you probably won't feel or hear much difference in how fast they clean up in the real world. The main difference you'll hear will be with the pots on 10.