Well, we haven't been to Greece, but been in Paris/South of France in late Sept to mid-Oct, and in Switzerland for 3 short nights in early Oct. Weather was great for both. (Well, except for some torrential rain/flooding in the Cannes area, but we were in Provence at the time - experienced some local flooding).
I thought south of France just amazing at that time of year - we were in NiceCote D'Azur mid-Oct and the temps felt like July at home, but without the humidity.
I'd love to get back to Switzerland as well...and Greece is on my bucket list! Not much help, am I? ;)
I'd prob do Greece, but just because we haven't been there yet (as the previous poster said). If you haven't been to any of these places, it would be so hard to decide.
I can comment that as beautiful as Switzerland was, it was expensive. We lucked out because we couchsurfed, so didn't pay for accoms, and our hosts fed us breakfast and one made a nice buffet, so we didn't eat out much, but the food was really expensive. Hubby got a hot dog and fries from a street cart in Zurich - compared to what we'd pay here at home for street food, it was about 3x the price.