I've gone on the Saturday in the past and it was busy but fine. I'm sure Friday will be quieter if it makes your life easier just go on the Saturday. Transport there is dead easy but just make sure you print off your medical certificate and get your doctor to sign it. Stick around on the thread. There's lots of people here who have run it lots (as opposed to my once) and so have really good info.
EdinburghLoon I'm not sure it matters much. I'd go with whatever you feel most comfortable with. If it's possible for you try to get there on the Friday so you can avoid rushing around on the Saturday. It means that you can eat well and relax on the Saturday instead and take in le Parisien atmosphere.