It depends on the actual aircraft you want to represent. The Copper State kits each show different mounts for the various schemes in the kit. It's been a while since I had the Eduard 1/48 versions, but I remember they also had more than one type of mounting in each kit or photoetch set.
One of the choices of mounting in the CS Nieuport XVII Early kit looks similar to the one on Guynemer's, and the other is a somewhat "simplified" version of that mount. The Foster mounting looks like the one included in the Copper State RFC Nieuport XXIII kit. Those are the only ones I own.
A photo of the subject you want to model is the best way to tell what is "accurate" for that particular machine.
Remember, this was all new, and a lot of experimentation was done to find the optimal way to arm these aircraft. Often, it even included modifications at the local unit level, or even by the individual pilot.
Pete