This guy ^^^ knows what he's talking about. Great job of summarizing the differences between the two lines without writing a novel about "the high-mid presence was palpable", "the chocolately low-mids enveloped me without devouring my soul", etc. I agree that ML should make a better center than the 50XTi. I'd like to see a $1750-$2500 center that matches the 60s.
We carry both lines. The Motion60XTi are one of the best speakers I've heard at the price point for many types of music. However, sound is subjective and the type of music you are listening to makes a ton of difference. The section of Dazed and Confused - Live at MSG 1973 from 4:04-8:23 that I was listening to 2 hours ago on my ML ESL-X models sounded much better to me then the same content on the Motion60s. However the Deftones, Korn and DMX I listened to right before this were better on the 60s. The midrange and upper bass presence (also noted above by guy that knows what he's talking about) on the 60s is fantastic.
All...measurements, shmeasurements. Choose what your ear wants and that choice will suit most things you listen to as all speakers have compromises. Bring lots of demo material that YOU LIKE to your listening appointment.