I've never heard Sig's but understand they sound REALLY GOOD.
I have heard 20's. Could you do a little comparison for us? - Thanks
Seth and Jerry give good information. This is the Beryllium tweeter.
For my ear, the Beryllium tweeter affords extreme high frequency extension in a very open and clear manner. This is most easily observed on sounds like cymbals and chimes. For me this was pretty important. However, I have a friend who doesn't think the difference in the treble between the Beryllium and the aluminum tweeter is that significant.
As Seth indicates, if you plan to pair the speakers with a sub, I think the S1 offers better SQ. The rigid ribbed aluminum cabinet is very much free of coloration and allows for a larger mid-woofer to be used than you would expect in such a small case. The sealed cabinet design gives it is very smooth roll-off. The S1 is less efficient than the Studio20 (84dB for the S1 vs 87dB for the Studio20).
Among the speakers I have heard, and as a stand alone (no sub), I think the Studio20's offer a great balance across the board for a speaker of their size and cost.
Both are built like tanks and seemed comfortable playing as loudly as I can stand in my Living Room.
Hope this helps.