Hey there, I can't say anything about Nice myself, as I've never been there, however I have been living in Rennes for the past year and can vouch for it's suitability for easy social life for 20 to 30 somethings (the range which myself and my friends fit in). I've had some friends take the French classes as CIREFF and say good things about it, though it is very intense apparently.

For me and a lot of expats my age the centre of the social life here is the local Irish pub. It's a bit of a cop-out as far as integrating and speaking French as the staff are british/irish - but even going there exclusively you'll meet lots of French people. There's a pretty decent night life here, it being a student town, but it's relatively relaxed as well, being small enough to walk almost everywhere without taking the bus or metro.

If you want any specific information on Rennes let me know.

Glen.