I had the 50mm Distagon (SLX version) and then upgraded to the 50mm FLE PQ a while back. The newer lens is sharper in the corners than the older lens. I usually use mine for landscape between 4 meters and infinity much of the time, so I keep the ring there most of the time and don't have many images made with the ring in the closer positions. Overall I'd call it a fantastic lens.
I have two 40mm, the older Distagon (SLX version) and the newer 40mm/3.5 Super Angulon, so I can't comment on the FLE PQ.
The 40 is pretty wide, I use it mostly indoors, rather than for landscape. I'd pick the 50mm FLE if I were you.