A P with flats gives you "that" sound. Countless recordings and the archetype bass sound in most of our heads.
The SR5 with rounds would serve the characteristic nature of that bass best; an active, powerful, grindy sound.
The most obvious solution is flats on the P, rounds on the SR5. You would probably get the most sound options that way as well.
But sometimes, doing exactly the opposite of convention is the way to go.
What kind of music do you play?