Beware: pre-paid devices (including USB sticks) are usually SIM locked for 2 years after you buy them. That means, if you buy a pre-paid USB stick in France, it will most likely be locked only to be used with SIM cards from that particular French network for the next 2 years.
I'd suggest buying a un-locked USB stick on eBay and then just buy a pre-paid SIM when you get to France.
PS-I have a
new unlocked E172 USB stick I'd be willing to sell for 100 CHF (SIM card and airtime not included), if you're interested, PM me