I got a card for a trip to Paris about a year and a half ago. After a lot of research, I ended up getting one from LeBara - ordered it online and had it shipped to the hotel a few days before I got there because they only ship to French addresses. It was waiting for me when I checked in. I remember i had problems activating it but eventually got it to work (if I remember right I couldn't manage to activate it via the website, I had to use text messaging on the phone).
But honestly, if I had to do it again, I probably wouldn't. All the hassle of getting the card and activating it wasn't worth it. I really didn't use it much, even with our group splitting up a lot. I could have survived on wifi, and just pre-arranged meeting places and times or the default plan of "meet you back at the hotel sometime before dinner!". The data was only 3G so not very fast.
The services that will get you a SIM while you are still here in the US are outrageously expensive for what you get. So I'd avoid them and either just order a SIM to your hotel, or pick one up at a shop when you get there if you are determined to get one.
My experience was with an Android phone that I knew used bands also used in France. You need to check out whether or not your US phone will even work with the services there, I don't know about US-carrier iPhones. They probably do, but you need to verify. (Just because the French services sell iPhones doesn't mean yours will work - different versions of the same phone will use different bands.)
ETA: Also check with your US service - mine would have worked in France for texting and calls, it would have just been a little pricey with the particular plan I'm on. But for just a call or text here and there, it probably would have been cheaper. I thought I also needed data, so that's why I went ahead and got a French SIM - in my case, my US data plan wouldn't have worked. But yours may.