I don't think
Marguerite is at all old-fashioned. I think it's quite lovely, but I wouldn't use it myself. (My former supervisor at work is named
Marguerite.) The only thing that bothers me is the -gue- sound for some reason. But it's just a matter of pronunciation taste.
I think
Margot is pretty "now" at the moment. It's cute; I like it. I don't know that I'd personally use it, but I do like it more than
Marguerite.