pots with 250K will sound warmer than pots with 500K
and the 500K pots is hotter and brighter than 250K
so.. the 500K is producing more output than 250K
usually single coil PU use 250K pots... but u can experimenting using 500K for more power and more high tone
and humbucker PU use 500K pots .. the humbucker itself actually lost some high tone, because the 2 coils cancel each other, cancel the hum, but it produces more output and nice warm tone .. so to regain some high tone.. 500K pots / 1000K pots is used ... if u want to experimenting.. u can use 250K pots if u felt that it sound bright
P90 maybe 500K pots... P90 is a single coil but its been wound like humbucker , more power than regular Single Coil like strato has(p90 has the number of wounds around 9000-1000 in one coil, but humbucker has +/- 4500-5000 wounds or maybe more in each coil)