Re: I should know this...
All comments are correct. It's a personal preference for sure, but generally speaking the 500k works best with buckers and the 250k works best with singles.
If you already have a 500k pot, I would try that first and see how you like it. If it's too bright/shrill with the single just change it to a 250k (assuming you have two vol pots). If you currently only have one vol pot and it sounds good with the bucker but too bright with the single, then beaubrummels suggestion for a 500k/250k concentric pot it spot on.