An OAG makes sense if you own a scope with longer focal length. On shorter focal length a guide scope will work well. My neighbour owns a 200/1000 newton and is fine with a guiding scope. He has no problems with flexure. With my 1624 mm i had lots of problems with differential flexure as seen here:
Differential Flexure
As i switched to an OAG all my problems where gone. It is more challenging to find a guide star, but - and here we are back at your actual question - the 120MM-S is a beast of a guiding cam. I can fully recommend this cam as a guiding cam.