Theoretically, I would expect P90s to hum more because they have more turns of wire on the coil. Coiled wire is an omnidirectional antenna, and having more turns makes it more efficient at picking up signals. Most P90s aren't shielded (except ones with nickel covers), P90 structure is very similar to Fender-type single coils, except the magnets are underneath. In Fenders, the poles are the magenets. Experientially, P90s I've had can hum just as much if not more than Fender single coils. The P90s in my SG Special and Casino both hum about the same, despite the Casino having nickel covers. And I had Phat Cats in my Les Paul for a while but they hummed so bad I sold them after a week, and those were covered. (They sounded superlative, though. If I could get that sound without the hum, it would have been the perfect Les Paul.)