I've gone thru quite a few P90s, so I'll give my input.
General notes:
• In general, P90's are a naturally midrange heavy pickup. Overwinding them gets you more output, but they can get honky. Anything over 9.0K starts to sound kinda crappy, if you're using vintage guage wire.
• P90's are also one of the noisiest types of pickups. Expect High-output P90's to have more noise.
• 8K is the Gibson-standard output. I prefer a slightly hotter bridge, and a pretty underwound neck at about 7-7.2. Underwinding the neck P90 gets you more clarity on top, less mids, and less bass boom. I prefer cooler neck P90s.
• In my experience, P90's need a decent amount of output difference (calibration) in order to balance in the neck and bridge. Gibson's standard setup is two 8.0K output P90s. They don't make calibrated neck and bridge positions. While you can balance them with height alone, one position will always suffer.
Different brands:
Duncan: The Vintage P90 set is a super bang for your buck set. Nice warm vintage tones, but still plenty of kick in the bridge. The Hot and Custom are also nice, but very thick sounding. I currently have a Duncan set in my LP SPecial that is 7.2K in the neck and around 8.8K in the bridge, and it's perfect.
Gibson: the Gibson P90 is a great pickup. More "clang" then some others. the only problem is calibrating. See above. I have one of these in my single pickup LP Jr. Rocks hard, sounds raw, and since there's only one, no need to calibrate.
Lollars: Lollar is one of the only manufacturers who uses a difference construction technique. He forgoes the baseplate. That combined with his special wind and magnets yields a P90 that has more clarity than others, while still sounding raw and vintage. I had a set, and regret selling them. Not quite as "barky" as some of the others listed; a little string-y-er on top, in a good way.
Fralin: Fralin's design and construction is very true to the original Gibson design, and he will calibrate them to your taste. Great tone out of those. I had one that was I believe -10% in the neck and +5% in the bridge and it was awesome.