Montepulciano or Pienza

Pienza is much smaller.
That means easier/quicker to arrive-depart in your car, also means less immediate area for walking and less restaurant options.

Both have great views. Pienza's main product is Cheese ; Montepulciano is Wine.

I choose Pienza as we also only had 3 nights, I thought we made the right choice as I wanted to be a little closer to the Val D'Orcia famous landscapes between Pienza and San Quirico.
If that is not a concern and/or you are a wine lover than Montepulciano may make more sense for you.

If staying more than 3 nights I would definitely say Montepulciano, after 3 nights in Pienza we had seen every inch of town a couple of times.

We stayed nearby in a hotel (no non-hotel food within walking distance, teenagers would maybe be bored there, despite the quiet, pleasant outdoor pool) in Sinalunga. We like to go to bed by 10PM, so we often have our large meal in a country like Italy or Spain at lunch, and only snack at dinner time - before all the best places are even open for dinner. We liked Pienza a little better than Montepulciano, but as I said, we didn't sleep there. I always worry about street noise, so we liked our small luxury hotel (Locanda dell'Amorosa) with no noise sources for miles around. In fact, we needed (August, 2016) their fierce air conditioning, which also keeps things inside quiet. Opinion.

I've stayed in Montepulciano twice and enjoyed it both times. It's big enough to provide variety in dining and other activities while maintaining a small town feel. I prefer the lodgings in the upper part of town near the duomo, where it's more level. Some lodgings, such as Meuble il Riccio, have nearby parking spots available. Pienza is charming but tiny, and for me there were too few evening options to warrant a stay. That said, one advantage of Pienza is that it's flat, whereas Montepulciano sits on a steep incline.

Since you'll have a car to use for exploring, why not consider staying at an agriturismo out in the countryside? It may be too early for swimming in a pool, but there may be more activities to appeal to your teenagers. As mentioned, although Montepulciano is larger, it is geared around wine.

What interests your teenagers? Another possibility in another part of Tuscany is Volterra, especially if they were into the Twilight films (never seen them). It's a charming town with Etruscan and Roman ruins. There's a promenade outside the city gates where the locals gather in the late afternoon. It's great for people watching. There's also enough cafes and restaurants for the length of your stay.

Thank you all so much for your helpful replies.

We're leaning more towards Montepulciano simply because of availability. Since I posted a few hours ago, we've managed to find somewhere that we're very happy with. None of us are wine drinkers, but some of us do love cheese. Would it be weird if none of us drink wine?

We've had a hard time finding accommodation in Pienza.

Until yesterday, we thought we were staying near San Gimignano, but decided against it.

Volterra would have been nice, but we couldn't find much there either. I may look again if I have the time and patience!

Our teenagers are fairly easy. As long as they have good WiFi, they're good to go! One of them will enjoy the pool if it's warm enough.

I've stayed in Montepulciano twice for 3 nights each time, and appreciated the restaurants and strolling around that are so much a part of evening life there. Both times stayed in the Politian apartments https://www.politian.com, up near the top of town. If you change your mind about where to stay in the Montepulciano area, I suggest considering these. There is plenty of room to spread out in the apartments, parking just outside the front door, and, yes, wifi! But no pool. The website is accurate, in my experience, regarding the layout and furnishing of the apartments.

Montepulciano. Stayed there twice times in the upper old historical area by Piazza Grande and it was wonderful. Only a short ride to Pienza for a day trip. Recommend Camere Bella Vista or the Hotel Duomo.

Not sure if available for your dates but this place in Pienza has a really nice pool!
http://www.anghelhotels.it/relaischiostrodipienza

It is probably the only hotel in town with a pool.
Restaurant is good too, oddly though the restaurant is not owned/managed by the hotel except for breakfast.

Thank you all again. Montepulciano it is! We're looking so forward to it and appreciate everyone's helpful replies.

As others have said stay in Montepulciano and visit Pienza. Take a couple of hours and drive down the backroads, if they are gravel roads so much the better, you'll enjoy it. La Botte in Montefollonico was our favorite resraurant.

Pienza is safer.
People in Montepulciano seem to be terrorists who like hand grenades lately.
I wish Rick Steves warned his readers about these things.

http://firenze.repubblica.it/cronaca/2018/03/19/news/montepulciano_ritrovata_una_bomba_a_mano_davanti_alla_caserma_dei_carabinieri-191663650/?refresh_ce

Roberto, good grief!

Our experience is that which ever town you stay in you will spend a half day exploring the town then you will hit the road to explore other places. It's not like you are signing up to spend most of your time in one place.