Hi Ian,
we went to La Grande Metairie in 2002 & 2003. It's a very nice site, there's plenty to do and it's got a lovely warm pool. Just make sure you're not pitched too near the bar area as it can get a bit noisy in the evenings. There's a decent beach about 10 minutes drive away at Les Conches. Arrive well before 1pm as it gets very busy after that. Hope you enjoy it as much as we did.
Don't think I can help you with Biarritz. The site we stayed at in Lesperon was nice & quiet, & very clean but didn't have too much for the kids: just a small pool, small play area and a table tennis table. Our kids made their own entertainment. I think their website is "parcducouchoy.com" if you'd like to have a gander. We'll probably return there once the kids have grown up.
Best of luck
Mark