Martin Logan Floorstanding Motion Speakers: 20 or 40?

Hi folks, 

I had a question for those who've experienced the Martin Logan Motion speakers. I am looking to purchase a pair of good floorstanding speakers. After reading through reviews of various speakers (Klipsch Reference Series, Infinity Reference Series, ELAC, Bower and Wilkins 685 S2) I called the local Magnolia home audio expert. He recommended trying the Martin Logan Motion 20 speakers. 

I went back and read the reviews on these. They are generally quite favorable and I'm willing to take the plunge. My question is: should I go with the 20s or the 40s? I live in an apartment and don't want to crank up the volume too high. At the same time, I would like to purchase speakers that led me enjoy hi-fidelity, clear music at a lower volume with adequate (but not booming) bass support. If anyone has alternative recommendations, please feel free to share those. 

I already have a Klipsch quintet home theater system and will be looking to replace the two front speakers with the floorstanding ones. I use a Polk Audio PSW10 subwoofer and a Sony receiver. (The receiver will also be replaced by a Marantz receiver; either the SR5010 or the NR1606.) 

Thanks for your time!