Discounting the fabric advantages or DFC support of the sup720, my understanding would the supervisors would be on par for "traditional" L3 ACLs & VACLs. Where the PISA comes into its own if you want to do NBAR (network-based application recognition) or FPM (flexible packet matching).
The PISA appears very attractive where you want the capability to deeply analyze traffic. The sup720 has more potential power for traditional switching/routing.
For a possible "ideal" design, PISA sups would be on the edges where you need the initial traffic analysis, and behind them, the sup720s. I'm guessing your facing an "either or" and not a "can use both" issue. If so, this is a tough call especially since the choice will also impact line cards.
If the choice was just between the original sup32 and sup720, then it's much simpler. Since the sup32-PISA is on the table, assume someone considers what it offers important.
What you might consider, if you can not do both (PISA and sup720), is something like a PISA on the edge with something other than the sup720 on the inside, such as a 4948, 3560-E or 3750-E.
