boy - none of these comparisons are apples to apples -
first off - the 1.2 OS weighs about 30 ozs- and is one helluva good glow engine -
the YS weighs a bit more - is more expensive but more power

The closest in weight and power is the RCS1.40 -and on a pipe is equal to the 1.2 OS and YS (on a 16x8 prop ) cost is also highest.
The new Zenoah -if you don't mind the side carb and clunky muffler and muffler (?) is a bargain.
For many sport flyers and that is where a lot of flying is done - the Z26 is probably the best buy .
But if you are retrofitting a pattern model and like the smooth cowled in look - the RCS is my choice-
We helped a friend setup one on a 10 lb EXCESS and it is a perfect setup -as a pattern model.
quiet , powerful, cheap to feed.