They will both work. The 500kohm higher value pot will have less loading to ground. This will tend to cause the resonant peak of the pickup to be a tad higher, so the pickup will sound "brighter" or "spikier" or "edgier" or "growl-ier" or "punchier," depending on where the resonant peak of the pickup is. By contrast, the 250kohm pot lesser value pot will have marginally more loading to ground so that the pickup will sound "smoother" or "mellower" or "linear." The effect is subtle, and depending on the rest of the signal chain, from fingers to strings to instrument to amp to cab, it may or may not be noticable.
You don't get "ramp" issues unless you use really high, like 1 meg, or really low, like 200kohm or less, pots on standard Hi-Z passive pickups.
What I like to do for most applications is to bridge the hot and wiper lugs with a 1meg resister to keep the definition from the higher value pot and less loading to ground, but modify the taper so it's a little more linear at the top of the knob travel, effectively a 330kohm hybrid curve, and flatten the "ramp" out just a little bit more.
Likewise, if you like the ramp of a 250 kohm pot, but like more presence in the tone, you can always put another 250kohm resister between the ground lug and the ground, and emulate the loading of a 500kohm pot.
And we haven't even gotten into balancing pot values and tapers for tone controls, which would take up another sizeable chunk of bandwidth. Suffice to say a 500kohm linear pot and a .033 capacitor as a tone cap on any moderately wound pickup gives much more useable shading than most stock setups, as the ramp is flat and the hinge frequency is just a tad higher than the standard .047, but doesn't get into guitar territory with the .022.
