Visit Montepulciano or Montalcino en route to Assisi

We stayed 3 nights in Montepulciano. I'd say it is worth a visit, for sure. You could also day trip back from Assisi if you have an open day. Closer to Assisi is Cortona which is more along your route (based on Google maps). That is another nice hill town (it is also the town from "Under the Tuscan Sun"). We spent half a day in Cortona from Montepulciano. There is also the Perugia Chocolate factory along your route. We did that too, and while it wasn't spectacular, it was something different and we got free chocolate!!!

It is possible, absolutly.
My first choice would be Montepulciano. Just more charming in my opinion.
And agree on Cortona, nice, relax and charming.

Montalcino is too far out of way, but Montepulciano is an easy detour and imo the more interesting of the 2 towns. Your friend should look for a parking spot along the road near the top of Montepulciano, as it's a long, steep walk up from the bottom.

I had a very pleasant surprise, visiting Montepulciano. The origin of our trip was to see The church of Saint Agnes (bourne 1268) my wife read about and I told her we can go visit because I bet is still there. Told her, you can walk the same streets, clime the same stairs, touch the same walls what she walked and touched, back in the 1200's (In Europe we have a tendency (she's American, and i am European) to keep places or rebuild it, as much as we can, after the devastations of two World Wars) So I know it it will be there guarantee, and after a google search of the church we packed our bags and left.

So.... I want to see the building and the town, not knowing how magnificent this city was, with all the specialty shops, coffees, wine shops, and restaurants. Almost fainted by the excitement when walked through the city gate seeing the town.

We stayed in the heart of the city, and I will add this place to my narrow list of: I got way more than I expected!!!.
Definitely go to Montepulciano, (we are heading back in May again)

I would also spend a few hours to a full day in Perugia. It has a compact old centre and I liked it a lot. Park down the bottom near the station where it is much easier to park, and catch the mini metro right up to the top station to get wonderful views and then wind your way into town. The main square is a delight and there is an aqueduct as well in the side streets. Great views from this lovely city. I think with three whole days you could easily see Assisi AND Perugia.

Montepulciano is closer, a couple of hour drive. Montalcino, having better wines (although some would argue) is about an hour plus to the drive. Stay at Villa Poggiano (we have many times), it is wonderful, outside of Montepulciano, and a great find. Montepulcian would be my choice (and staying at the Villa).
enjoy
John