The older f3.5 135mm and 28mm M series lenses are among the sharpest lenses Pentax ever produced but... they are rather slow (small aperture) compared to other prime lenses. I don't think you will find a significant difference in image quality but you will want the extra speed occasionally.
When you have 2 lens of the same performance with one just slightly faster. which one you are to put in your camera bag when you head out in the morning? The faster one.
There have been some discussion in the other forum about using the M135mm3.5 on its merit. It lags behind its very prominant brother FA135mm2.8 by just half a stop and it is manual focus. A lot of us just put it away as it does not hurt and you do not get that mucheven if you sell.
I am sure this lens is stellar in performance when stopping down to 6 or 7 or outdoor.
Well we are spoilt.