Fly Delta (operated by Air France) or Air France from Florence to SFO

They are codeshare partners, so either airline will supply the aircraft. If you have a mileage program with one, book through that one.

I would avoid any form of transportation operated by a French company with French employees (as much as possible) because of their horrible tendency to strike and strand passengers. Sometimes it's not possible to totally avoid it, but do be aware you might end up without transportation.

If this is how it's worded ("Delta operated by Air France"), then these would just be two names (and flight numbers, one DL and one AF) for the same flight, just depends on who you buy the ticket from. As Zoe noted, the only possible advantage to one or the other would be if you have miles or status with Delta or Air France.

And, the Germans are no slouches at stranding passengers due to a strike, either.

Delta and Air France are partners within the Sky Team Alliance. Their flights will be in codeshare.

I've traveled from SFO to FLR many times and I have purchased the ticket with either Delta or Air France depending on which of the two had the lower price at the time. No matter which of the two you buy the ticket from, the flight is in code share and it will be operated by Air France, both the leg from SFO to Paris CDG, and the leg from CDG to FLR. Delta does not operate flights from SFO to CDG, so even if you buy a Delta flight, you will be on an Air France B747 from SFO to FLR. The short leg from CDG to FLR is also aboard an AirFrance Airbus318.

I have flown Air France from SFO to FLR numerous times, probably more than anybody on this forum, and have no complaints. I actually don't dislike AirFramce at all. Unlike US based airlines, Air France will offer you plenty of free alcoholic beverages for free on that flight. I gladly take advantage of that perk to the fullest extent possible. Food was decent too. The connection sometimes was tight and CDG is a huge airport that is not the easiest to navigate. Occasionally I missed the connection when my arrival at CDG was delayed, but AirFramce put me in the next available flight to FLR (there are 6 a day in each direction) and they even gave me coupons for free dinner and free drinks for the inconvenience (not sure they do that anymore, but for sure US airlines didn't do any of that even then). I've never had the opportunity to be left stranded by AirFrance due to a strike, but once, due to weather, my flight back to SFO was canceled, so I was stranded in Paris at Airfrance's expense for a day and a night. It may be have been a slight inconvenience but I'd rather be stranded in Paris than Atlanta, GA or Jamaica, NY.

In this case it doesn't matter which company issues your ticket because you are leaving from European soil, but one thing to consider is that European law protects you more fully if you are delayed or stranded. For example, during the two weeks planes were cancelled due to the Icelandic volcano, the European airlines found lodging for stranded customers. The US carriers didn't. For that reason I opt for Air France planes over Delta when the choice is available leaving the States. All companies are subject to EU law when leaving Europe. The free liquor is nice too!

Roberto got the dinner and overnight stay paid because it's the European law. I've had food served to me in the terminal by Air France when the delay hit two hours--it's the law.

Also, if you want to pay for an upgrade, it's easier to deal with the company actually flying the plane.

What Bets says is true.
If you fly on a US carrier, you are not entitled to be covered for expenses if the delay is not due the airline (e.g. an act of God like weather). So if you miss your connection to Europe at JFK due to weather, you are responsible for hotel costs. That is the law in the US.
But if you fly with a European carrier and miss your connection in Europe, as it happened to me three times with AirFrance and Lufthansa, even if it's due to weather, the European airline will cover your hotel and meals costs while you are stranded there.
That is one of the reasons why I never fly to Europe with a US based airline unless it is a free mileage award ticket, or if the price is hundreds of dollars cheaper. In your case it doesn't matter since you will certainly fly an AirFrance flight if you fly straight from SFO to CDG (then to FLR). AirFrance will cover you if you get stuck in Paris, even if you purchased through Delta website.

Little confused about the talk about getting miles for flying. Whether I buy from delta.com or klm.com or Expedia I will get the same number of Delta miles for flying on Delta.

Although true with Delta and KLM, and presuming we are discussing redeemable miles, if you purchase a ticket from Air France and you put your Delta Skymiles number onto the reservation, you will earn miles to your Delta account based on distance flown. If the ticket is bought from Delta, the miles earned are based on how much the ticket cost. So, in the OP's situation, there could and probably will be different mileage earnings depending on which airline the ticket was bought from. It's all spelled out on Delta's website, so sometimes a little research will get a person more miles one way or the other.