I will answer my own question since I found some good analysis of penetration depth/density from some SPE papers.
For my project if I was perforating at 20spm the biggest charge I could use is 23gram. If I perf'd at 17spm I could use a 25gram charge which gave me deeper penetration. So basically it came down to penetration vs density.
Unless I am perforating thin zones I have have been using the bigger charges @ 17spm for all my latest completions.
Here are three of the SPE papers that I looked at. Mostly they talk about using a larger charge to penetrate past the near wellbore damaged zone. However there is a bit of discussion about reservoir properties.
There's a Schlumberger software called PURE (I think) that lets you play around with shot density, phasing, charge size and so on (using their charges, obviously) that is pretty useful, and is (or was) available for free. Talk to the nearest SLB salesman!