For other people's reference, to answer my own question: I had a wonderful experience in both stores, and I think both are worth visiting for differences in stock.
As PoshHoney pointed out, FSH cannot check the stock in George V, so it is definitely worth checking the shop itself-- I found exactly what I was looking for there, and they did not have what I was after in FSH.
The lady I spoke with in FSH was friendly and helpful, and very knowledgeable, as well. The lady in George V was exactly the same-- polite and friendly, very willing to help. I had some minor credit card issues while there (I had very stupidly forgot to tell my company we were going away, so when they tried to put the charge through the card froze and I had to call them), which was handled very well by my SA, although the guy behind the counter didn't seem to be too impressed with the holdup. While I was speaking with the CC company, mum said that she felt a little pressured as we were the only two people in the shop and she counted no less than ELEVEN sales assistants.
The stock bag-wise is probably a little better in George V than FSH if you are looking for a Birkin, although if it's a Kelly you are after FSH will probably be able to help you out. FSH also has a much larger selection of stock generally, particularly in scarves.
Overall, the stores are quite different in what they have, in my experience, so it's definitely worth going to both to see what you can see. They aren't that far away from one another (I think it is about five metro stops, or under €10 in a taxi, which was how we got there) and when you are asking for the George V stop in your taxi, be sure to include the street number because the taxi drivers do not seem to know anything about where the Hermes stores are.
