My response might not be as relevant as others since I haven't been to Greece yet but we're planning our trip there for this spring so I've been doing a ton of research. However, I've been to France 3 times. I've also been to Italy and many other countries in Europe, except for Greece. I love France! As others have noted, there are so many diverse areas of the country to explore. Each one is unique, charming and well worth visiting. And the food and wine are simply amazing.
Contrary to the common French stereotype, we've found the French people to be wonderful. Honestly, the most helpful people I've ever met. For example, we were standing in the Metro in Paris trying to figure out which side of the track we needed to be on and someone came up to us, totally out of the blue, saying, with a big smile on her face, "Follow me- this way to Notre Dame!!" We didn't even ask for help (we must have looked really lost!).
Even better was when we were at the Nespresso store on the Champs Elyses trying to buy Nespresso pods from the automatic machine. Unfortunately, it wouldn't take my credit card. The line was backing up behind us so we decided to just leave. We headed out the door and a woman came running down the street after us, saying she'd get us the pods with her credit card. We walked back with her but at that point, someone else had already stepped up to the machine. This woman simply elbowed her way back in, asked me which pods we wanted to buy and proceeded to get them for us! I offered to give her cash but she wouldn't take our money. Can't say I've ever had someone do that in the US. ;-)
Every part of France is wonderful, especially the Loire Valley, Provence, etc. As someone else mentioned, the fast trains get you wherever you want to go, efficiently and inexpensively.
Given what I've read about Greece and learned from friends/family who've bee there (my brother lived on Santorini for a few months), I expect to love Greece too. I've heard wonderful things about the people, the food and overall beauty of the landscape. Each European country has something that makes it special and I have yet to visit a country I didn't enjoy so I truly think you can't go wrong either way.
Sorry I'm not more helpful but I wanted to put in a big plug for France. It's my favorite country (so far). :-)
One thing I guess I would consider is time of year you're going. I hear Greece is very hot in the summer. France is much more temperate. Maybe that will help you decide!