I agree that 500k generally works just fine for all high impedance pickups. It is not worth freaking out over, anyways. It makes a subtle difference in most cases.
Brand makes no difference. Manufacturing specs make all the difference.
Tolerances are generally 20% on most pots. If this bothers you, you can buy something tighter, but what most people do is just grab a handful of pots and pick out the ones with the best tolerance.
The rotational inertia of the shaft tends to be a personal preference. Some people like very stiff pots that don't move, others like them a little looser. It makes no difference electrically, however.
With regards to the composition of the resistive element, it makes little difference in this application.
Pots rated for a higher wattage can generally be said to have a lower contact noise, due to the larger resistive area, but this is also not significant in this application.
Taper is of great importance. Many audio taper pots are not actually audio taper, but rather, a combination of two linear segments in an approximation of a logarithmic curve, in order to cut manufacturing costs. Look for pots with decent tapers.