Well, we are a couple in our mid 30s as well. No kids here either.
We have stayed at all three. And, it really depends upon what you want. I know you said "quiet." But, what are your other plans? City Walk at night? Do you like to have a drink before walking to the parks? Lots of pool time? Mostly time in the parks?
Your perception that the HRH is loud and overrun with teens is absolutely false. You will find families at all 3 resorts. We loved the HRH and will have stayed a total of 15 nights there this year alone. The HRH is a hip place, but there were times we walked through the pool to go the parks and no one was at the pool, and we didn't see a soul beyond the pool/bar staff until we got to the walkway. Mid-December when you are going is not a busy time at all, so I can't see the HRH being Spring Break/Christmas week crazy then anyway.
The RPR is undergoing renovations now and will be done by December. Some of the rooms may be a little worn, but the outside is still beautiful, and I love walking out of Tower 3, through the pool area, to the paths to the park. It really is a lush, tropical-feeling resort.
The PBH is quaint and quiet. But, our least favorite, actually. We stayed there when it was sold out, and the place felt like a morgue. Since you want quiet, you may like that. We would have a drink at the Thirsty Fish every night before catching the boat to the parks/City Walk, and we found all the bartenders to be totally aloof to the point we felt like we were bothering them just by being there. That put us off. Everyone has had a bad experience or other at some resort, and PBH is ours (we had front desk and housekeeping issues there as well). There are many who will not stay any place but PBH. The PBH does have the largest, nicest rooms, however. One more thing to add as to why we don't care for the PBH: the RPR has the Bula Bar and the HRH has the Beach Club. We loved sitting outside near the pool at either of these bars to grab a bite to eat and to have a drink. I don't recall anything like that at the PBH.