I owrk in a property company in Nice.
In my opinion, buy-to-let is much better. I don't know about Cagnes-sur-mer or villeneuve loubet, but Nice has a pretty strong short-term letting market at the moment. It should improve again late this year and esp for summer 08 - the tramway and all the roadworks/renovations are finishing this sept.
For Cagnes-sur-mer, I heard of a large number of people (mainly Irish), who got stung when they bought new-builds off a well known 'overseas property' company. I know the property manager for the developments, who was left to pick up the pieces and explain to buyers that they probably won't get 52 weeks out of the year at 700 euros per week.
Having said that, all you need is 12 weeks per year at seasonal letting rates to exceed the return from long-term lets. Most people get double this, or more.