Re: P-90
a humbucker is exactly that while a p90 is a singlecoil unless its a stacked p90. hum is inherent in a true p90. tonally, i find p90s right in-between a singlecoil and humbucker exactly what pfdarkside stated.
i love the growling mids and clarity a p90 has as opposed to a singlecoil and humbucker respectively. that said, there's nothing a p90 with a tone knob can't do. slap them in some good tonewood and it may well be the most versatile guitar you will have.