Hi Peter
I'd stick to the SPanish side, too. I've ridden a couple of times in the area and the roads just seem to be better.
One brief warning - last summer (2008) I went in a group of three bikes. The other two both got stopped by the Spanish police, with two on-the-spot fines handed out. They seemed to be focussing in bikes in and around towns and villages.
Some of the best GS-suited roads I've ever ridden are in Catalunya. The mountain roads are well worht a trip. If you want some examples, let me know and I'll try to dig out the routes. In reality, just pick a wriggly road and enjoy! We rode in the area roughly bounded by Barcelona, Lerida/Lleida and Tortosa. We stayed three or four nights in Tortosa Parc Hotel, very reasonable hotel, big garage - here's a link:
Hotel Tortosa Parc . Tortosa seemed to have a few decent hotels, though.
Slightly further south is a mountainous area called Maestrazgo. Look for a place called Cantavieja, and all around there is unbelievably good. It is very remote though (allgedly the least developed part of Spain) and while some of the roads are a little bumpy and patched, a GS floats over them while the less-enlightened riders strugglew on their sportsbikes - always excellent fun! In other places, the roads are simply amazing. Sort of triangle between Teruel, Morella and Castellon de la Plana.
If you're undecided on accommodation, and depending on your budget, think about Paradores. They're very good, you can stuff enough breakfast to see you through until dinner, and they frequently do deals where if you book several nights you get a reduction. You can join the loyalty scheme, Amigos de Paradores, which entitles you to more special deals. Not cheap, but all special. Here's a link:
Paradores.