P-90s are fatter than typical single coil pickups. They don't have that kinda hollow sweet sound that single coils can have; it's a bigger, stronger sound.
The thing about your quesiton is that there are varieties of pickups within the types you've mentioned. For example, there are vintage P-90s that sound really mellow, but there are also modern high-gain P-90s that can have a bigger and wilder sound than most humbuckers.
