Hey @airmaster, welcome to the HFGF !
I have owned a Pearl White, and also 909 (actually on my 2nd 909. I re-bought one after selling my first one). All in all the TH900mk2 Pear White is outstanding and “feels” a bit more sophisticated with the smoother more “balanced” signature while still being fun leaning. I like it for modern Jazz/Jazz Fusion, Funk and progressive variants. Where I want that fun but also want to enjoy the more natural (to my ears) timber and textures of instruments like saxophones/horns, drums, keys, and vocals, male/female.
The 909, though is definitely more “fun” to me bass is more prominent with little bit of sacrifice of textural quality, mids are not as forward as the anniversary tuning, but for me that makes things like electric guitars etc more enjoyable. I like these phones for Rock, progressive rock, electronic mainstream pop/rock blues etc. I don’t personally find the treble to get too harsh with either tuning - though, it certainly can point a finger at “lesser quality” recordings.
I have not heard the TH900 standard tuning so I can’t speak to that. My personal preference is for the TH909, but that’s just me. There is no right or wrong, one is not better than the other - it will distill down to you and your ears, your music library and the synergy with your gear/setup that will dictate as to which one is better for you.
You might want to check out our official Fostex threads for more info and observations.