"Nice" is a very individual term. Both towns are rather small, very old, and traditional in style, with restricted car use. Pienza is roundish, tiny, and flat, so it's the most popular with tourists. You can walk through the entire city in thirty minutes. Montepulciano is larger, more oblong, hilly, and it's built atop a steep hill. Walking through Montepulciano is a serious workout, and the walk could take some time. To come here with luggage and a car can present a challenge for some.
Accommodations in both places tend to be the B&B or full apartment type. There aren't many "hotels," not in the sense that we Americans think of a hotel. Reviews for the most popular lodging places are well documented. "Nice" can depend on your budget. Prices typically vary according to location, amenities, room with a view of the valley, etc.
There's no such thing as a modern, luxury property with numerous rooms. Everything is a variation on Italian traditional, leaning toward old but well-maintained. You study a map, look at the website photos, read the user comments, and decide from there.
As long as you avoid places designed for students, it's hard to go wrong on a lodging choice. The choices are what they are. One isn't necessarily that much "better" than the other, especially in the city center locations. I tend to favor places that offer a great view.