I really love my rigid Summicron on the 240, surprised you don't like it on the M-E. You do know that it was designed by Mandler who - before becoming Canadian - was visiting ELCAN on and off since 1952, right ?
In any case, if you really want to trade the rigid, I recommend the 50/1.1 Nokton for your purposes. Even better than the 50/1.5 - technically as good as the 50/2 Planar, but faster, see also
http://www.rangefinderforum.com/forums/showthread.php?t=153336.
Whatever you get, make sure you can return it, the difference between Planar, 1.5 Nokton, and 1.1 Nokton is so small that sample variation might well be bigger, meaning
try to get a good one.
Roland.