I think it's fairly finely balanced and I would lean towards 2x PG rather than 1x PP in your case. My reasoning is:
- Cheaper
- No status for your wife if she travels on her own
- Given there are generally 3 of you traveling and 2x PG gives you 4 E+ seats I don't believe PP gives you anything
- No ability to get 3 CPUs from 1 PP, but this can be done using 2x PG (although at a lower priority). I guess this depends if you want to sit upfront with your wife and kid in the back.
- Likewise with the 2 RPUs if there are always 3 of you traveling
- Similarly GE refund, not too helpful as there are 3 of you traveling (and I don't think kids can register)
- I think you're limited to one companion for the complimentary baggage rules, so 2x PG is again a benefit, but I can't believe you'd ever need 9x 32kg bags
- Bonus miles, 1x 25% + 1x 75% is the same as 2x 50% bonuses, and having balanced miles makes redemptions without needing to transfer miles easier
- The only real benefit I see for PP for you is the waived charges and you'll need to judge how often you would make use of this.

If you didn't have a child (or that child was still young enough to be on a lap) then it would be a different calculation. My wife and I just did these sums and decided it was better for her to shoot for Elite status on OW and my to retain PP so we were covered on either alliance, but there are only two of us and if she travels without me she's generally in paid C.