I'm looking at a few and at check out it ask "left or right side carry".
What difference would it make ?
Also fast flow valve or plug, I'm always worrying about the plug falling out.
Fast flow valve or plug? Well, that depends. You want more or less historically correct? Then the plug is correct. If you want more efficiency, the fast flow valve might be better for you.
Many people will tie the plug onto the horn with a short length of leather or sinew. Problem with that is a tendency to not insert the plug back into the horn.
Years ago, I always used a valved horn just because I didn't have to keep track of the stopper. Now I have gone back to just a plain horn with a 5/16+/- hole. It pours Ffg just fine. If you are using Fg powder, something around a 3/8" opening would work good. One of the brass goodies that PHM carries is a brass horn tip from Treso. Fg pours out of it like water from a hose.
Hope this helps.
John