We visited the French Riviera (Cap Ferrat, Villefranche-sur-Mer, Nice) a couple years back, and Lake Como this past month. I am a huge fan of blue water next to mountains and both locations are breathtaking. Can't go wrong with either, just depends on what you want to do. Keep in mind that as others said, the roads are not easy or fast. The tradeoff for spectacular nature is narrow, windy, tourist clogged roads.
For either destination, you should get on a boat. On the lakes, the 40 HPs that any adult can rent without restriction are good and speedy and you can see a lot. On the sea, you can also rent the 40 HPs, but the seas are rougher and the ride can be sickness-inducing for some passengers. My wife and I were fine with it, but our 25-year-old daughter got very queasy and had to cut her time short.
Lake Como temps are brisk even in summer, but not chilly. The sea is even warmer, and with the bonus of plenty of fish to see with your snorkel because rocky shores and pebble beaches mean much clearer water than sand beaches we're more used to. The fish aren't as pretty as in the tropics, but they're still cool.