Funny, as I have DR rounds on my P and Fender flats on my J.
I like the dynamic range of my 57RI P with its bright-sounding maple neck which keeps its definition when I roll off the tone using the rounds.
I like my 62RI Jazz to sound like it does on Jaco's Donna Lee and I use flats with the tone rolled off for that sound playing on the bridge pickup.
I think I prefer rounds on my P as its a bit of a slapper too, like me :-)