When playing press/3-2 base defense, they are almost interchangeable for me, and I'll move them around to exploit a defender matchup pretty freely. I'll usually default to better rebounding and block going to C, better speed and bh/p skills at PF. When playing 2-3 base, I am more cautious about moving them around, I generally want my highest low block defense in the PF position, and definitely want the top rebounder and shot blocker in the C position.
With my straight press team, I usually emphasize the speed tree, followed by rebounding inversely. Slowest, best rebounder at C. I keep all my ath/def on that team pretty high.