I have pressurewounds on a 5 string P and brite flats (actually the Smith branded version, slick rounds) on a 4 string P.
As others have said, PWs are in the round wound family, and BFs are in the flats family. I really like the tone of each, but prefer the feel of the BFs. Yes, a little sticky, but smoother than the PWs, which have a sort of coarse texture to them.
Tone-wise, PWs are like taking nickels to it's logical conclusion of fat and warm. Strong fundamental and low mids. Big sound.
BFs have a really nice woody character to them, and they will get some harmonic richness going. Great with light overdrive. Good mix of warm thump and clarity. Tighter down low than the PWs.
I wish I liked the feel of the PWs more, but I don't love it. Have a set of ghs balanced nickels on the way to check out, interested to see how they compare to these two sets.