Okay let's start out with the disclaimers - I'm a Sound Lab owner and dealer. I've owned several different Maggies, and I've auditioned the 20's but not the 20.1's. So please take my post with a grain of salt (or two!).
The Maggie 20's will be a little bit more efficient, and may have a bit more impact in the bottom end. My understanding is that their planar magnetic bass panels have somwhat longer maximum excursion than does the Sound Labs' electrostatic element (although the area is smaller). The Maggies will also take enormous amounts of power, and so will play louder than the Sound Labs - how much louder I don't know, but the older Tympani series would go north of 110 dB at the listening position. All of this in a smaller and less expensive package - the Maggie 20.1's are only 29 inches wide while the big Sound Labs are 35 inches wide, and the Maggies retail for $14,000 (I think) and the M-1's for $16,617 (with toroids and Cardas posts).
The Sound Labs have greater inner harmonic articulation and low-level detail, are more coherent (as one would expect from a single-driver speaker), and are more lively at low volume levels. Timbres and textures of voices and instruments are very rich and natural on the Sound Labs - this is a function of several factors, some of which are not obvious (I'll go into detail if you'd like). The Sound Labs also give you a wider sweet spot within which there is virtually no change in the tonal balance. In these areas the Sound Labs are arguably world-class. One note on maximum volume level - with the Sound Labs you can hear everything that's going on at considerably lower volume levels than what's required with most speakers, so in practice their 100 dB ballpark at the listening position upper limit (varies with amplification) isn't as much of a limitation as it might seem.
In my opinion, both speakers are way ahead of their box-bound competition, but then my personal priorities don't demand the very high volume levels that a good dynamic speaker can give you. Neither the big Maggies nor the big Sound Labs will have the chest-whumping bass impact of a good box woofer, but each will have better pitch definition in the bottom end than you're likely to find in a box even at several times the price. Both have very good tonal balance, although the tonal balance of the Maggies is somewhat level-dependent (in all fairness, this is the case with most loudspeakers). Both will be open and free from distracting colorations, and very relaxing long-term (I know from experience you can listen to the Sound Labs literally all day long with no fatigue, and expect the big Maggies would come very close in this respect).
Both speakers like lots of power, and both like a very lively-sounding amplifier, so in general amplification that works well with one also works well with the other. The exception would be OTL amps, which in general don't really like the 4-ohm load throughout the bass region of the Maggies.
I have two customers who auditioned the 20.1 and the Sound Labs, and both chose Sound Labs. On the other hand, people who auditioned both and chose the Maggies don't show up on my radar screen, so I can't reliably say that my two data points represent a genuine trend.
The good news is, I don't think there's a "wrong" choice between the two. Look at the volume levels that matter most to you, and that will probably tell you what the front runner is. But if at all possible listen to 'em both.
Best of luck in your quest!
Duke
