Having played both on more than one occasion, I would say that both are overrated. Having said that, part of the fun of being a golf dork like many of us, half the fun is the travel. You could probably enjoy yourselves at either place.
The whole Costa del Sol region is over developed and lacking any truly interesting golf. The culture and cuisine is excellent but if you're going to Spain for food go to San Sebastien. Why pay 330 euro, or whatever it is now, for a course you can play 100 times over in the states. I had been told how amazing the conditioning of Valderrama was, the Augusta of Europe? I played is several times while working in the area and every round had at least one ball plug in the green... not my idea of great golf. When I asked why, I was told that they needed to be kept soft because the greens were so small. If that is true, what a sadly poor design. I do love the Cork Oak trees on the course, although there are way too many of them! There are several other courses in the RTJ style in the area at La Reserva, Finca Cortesin, and the recently renovated Sotogrande. My favorite course in the country is definitely El Saler, I love that course although the conditioning could be improved. There are several really interesting design features, including amazing green contours on several holes.
I may be alone here in liking Oitavos Dunes, although I would love to see it play firm. Given the choice between the Sotogrande area and Estoril/Cascais any day. There are so many things to see and enjoy in the area beyond the golf. Sintra is spectacular and worth a side trip. On the same trip you could see other courses that might pique your interest as well. Penha Longa is a RTJ Jr. course not far away that has a moderate amount of uniqueness. If you don't mind a bit of a drive check out Troia. It was designed by RTJ, with help from Cabell Robinson. It is a fairly interesting design hard on the sea with some fun holes, although a bit tight. Or you could head north and see Praia D'el Rei, a Cabell Robinson design with several holes on the sea.
Why not check out Paris or Amsterdam? With Paris you could play Fountainbleau, St. Germaine, St. Cloud, Chantilly, and with a bit of help Morfontaine. With Amsterdam, you could see Noordwijkse, Den Haag, De Pan, Kennemer, Eindhoven etc. The positives are a lot less travel for much better golf. A plus is the dining in Paris, the nightlife in Amsterdam, or the art museums in either location.