Went in Nov 2013, day after dining at Joel Robuchon. Can't say enough nice things about both places, food is of course top notch crazy delish, service unobtrusively impeccable (seriously, I hate nothing more than waiters obviously watching me eat), and the ambience classy and expensive. That being said, the décor inside the restaurant is all nice and everything but they have no control of what's outside the windows. It was off-putting when I look out the window at my dining table and saw this huge 40-story tall ad of a showgirl wearing a sparkly thong. Her gigantic ass cheeks still haunt me to this very day...
I think Joel Robuchon has a nicer cozier atmosphere though, Guy Savoy felt more like somewhere you go if you don't know someone that well but still want to impress them. I'd vote Joel Robuchon for closer friends and family, and Guy Savoy for your boss and important clients.