The Yashica gives very nice images. The Mamiya's give great images.
The Yashinon lenses on 124Gs are capable of rendering outstanding images. And the difference between 2.8 and 3.5 is a mere 2/3 of a stop. Not significant in normal use, as most rarely expose with a wide open lens.
However, if one is looking for another advantage of any Mamiya TLR over any Yashica (or Rolleiflex), here's one. The Mamiyas were all designed with a straight-through film feed path. The Yashicas and Rolleiflexes were not. They both incorporate tight 90-degree film path direction changes by sharply bending the film over tiny chrome roller bars.
This design choice opens the possibility of leaving a partially exposed roll of film in-camera for extended periods* where it will take a set while under tension across that roller. Then when finally wound to the next frame, if the spacing is not precisely perfect (usually it's not for any TLR as there are no film advance sprocket holes), that "bump" can negatively affect film flatness if it falls within the film gate for the next exposure.
Some users of these roller bar TLRs reportedly wind two frames (wasting the first) for critical images, just to prevent this issue. I have seen this very problem occur in my 124G after removing a partial roll that had remained inside for a couple of months. But it is impossible for it to occur in my Mamiya over any length of time.
Many here will complain mightily about the slightly larger size of the Mamiyas without realizing why they actually are larger. What one buys for the price of that slightly larger body is the uncompromised straight-through reel-to-reel film path.
It's one of the main reasons I chose a Mamiya TLR.
Ken
* ...or less. One fellow APUG member has reported testing and seeing the problem in mere minutes. As in, between frames during the normal pace of using the camera in a single session.