I would say that Sahagún is a town, Valladolid is a city, but a small, manageable city. Sahagún has some marvelous mudéjar churches and some good food. A good pilgrim vibe. It has all the services you need, in terms of commerce. Valladolid has a very nice plaza mayor with the typical café set-up, a sculpture museum in an old convent with a Baroque facade that is impressively ornate even if you don’t like baroque (actually it’s called Plateresque, which is an over the top Baroque, if it’s possible to be over the top of Baroque!). Some nice churches, nice parks, nice pedestrian streets, a Cervantes museum in the house where he supposedly lived for a few years in the early 1600s. I think there’s a lot more to do in Valladolid, but that may not matter if you just want to chill out. There’s no camino connection in Valladolid, so you won’t be able to sit and watch pilgrims walking by as you would in Sahagún.