Taking a couple of extra days between Paris and Venice -- haven't seen southern France at all -- which area would you pick -- or somewhere else?

Definitely Nice; much more to see and do than in Aix, and Lyon isn't in southern France--it's up in the Rhône Valley.

They are all so different but for the south, I'd pick Nice. I love the old part of town.

Thanks! Please send any suggestions for Nice; I'm already checking the train schedules.

Lyon has far more going for it as a city than Nice, which is not my favourite place in France (overpriced and overrated in my opinion!).
However, the countryside surrounding Nice is lovely, both along the coast and inland (beautiful hilltop villages), and there is probably more to see in the immediate vicinity of Nice than there is around Lyon. It is certainly a great base for exploring the Cote d'Azur.

(BTW, Lyon is considered by many to mark the "beginning" of southern France, and this is reflected in its climate and architecture.)

Another vote for Nice, which has everything. A beach, a charming old town, the Chagall and Matisse Museums in the lovely suburb of Cimiez, great transportation to places like Antibes, the Villa Ephrussi Rothschilde, and St. Paul de Vence, nice flower and food and antiques markets. I actually think Nice is underrated. I'd choose Aix second and Lyon third. Lyon may be the geographic "beginning" of the south, but if you are looking for the Cote d'Azur-esque or Provencal Mediterranean thing, you won't find it there.