Main numbers to look at are your out-of-pocket maximums. Your employer contributes $1,500 so in the most worst case of scenarios for a given year, you are out $4,850. With the PPO, generally the OOP max is lower, but this can vary and sometimes the PPO is never the better option if the employer subsidizes the HDHP enough and it has a low enough OOP max.

For the sake of pretending we'll pretend the OOP max of your PPO is $1,500. If you have an illness, condition, or disability which will require you to be spending a lot of money on PT, medication, and surgeries, the PPO is of course the better option. If you have some $120/mo prescriptions which would be covered 100% under the PPO but you'd have to pay for under the HDHP, the HDHP comes out ahead. You get the employer contribution plus the tax benefits of the HSA.

If you're someone like me with no regular medical expenses (I have not had a hospital visit since I was 14 and I'm about 30 now) and my normal "expenses" are a $10 bottle of Ibuprofen or Acetaminophen every other year, my HSA is massively better for me.