John,
I have written about mediums a good bit and there is an article about them on
www.cybertours.com/~midnitebee/              click the lower right image

I switched to ALL Illinois (mediums) 16 years for a SINGLE reason - to have
just ONE SIZE FRAME in every box on the colony.  It had nothing to do with
weight then
because I was only 64 and still strong; but I was sick of never having the
right size frame and had to leave a hive of bees open while I went to my
honey house to locate a frame.  Frankly, I wish I had started in ALL ILLINOIS
68 years ago when I got my first bees in1933.

As I teach beekeeping today, quite a few are starting in all Illinois.  Out
west, they are called Westerlies and that is what many, many people use.

I hope I have helped.

George Imirie