I think the 100% covered maternity care on the PPO may be too good of a benefit to pass up. You may want to get clarification on exactly what that means though; is it just for consultations with the physicians and the birth itself, or are related services (blood draws, ultrasounds, etc.) covered under that as well?
With the PPO, assuming all the care you might need is covered under the 100% maternity care benefit, your total cost is $4,522/year in pre-tax funds.
With the HDHP, assuming the pregnancy will cost enough to blow through your out-of-pocket maximum, you pay $3,301 in premiums plus $4,400 to put enough in your HSA to cover the out-of-pocket max (after the $600 employer contribution), for a total of $7,701. You also get to save an additional $1,750 pre-tax above and beyond what would be needed for the out-of-pocket max. The tax savings here will reduce the difference between the two plans, but won't eliminate it.
The big variable here is how likely is it that either of you will need care that is not covered by the special 100% maternity care benefit that the PPO offers. In the above comparison, you've already blown through the HDHP out-of-pocket max, so any additional care you might need would be essentially free if you choose that plan. Under the PPO you haven't paid anything toward your deductible or coinsurance for the pregnancy, so any extra care you might need would narrow (and possibly eliminate) the cost difference between the two plans. The HDHP is essentially a known quantity: you pay a maximum of $7,701, a number you're reasonably likely to hit if you have a baby next year. The PPO starts at $4,522; any additional non-maternity care will cause your cost to go up, and these costs would need to be paid from post-tax funds (unless you have an FSA or a pre-existing HSA balance).