I was told today by someone who should know that there's no such thing as an Extremis 6s, though there is a wide BW Extremis 6.8, but it's an apples n' oranges thing in that the FR125S is a huge wide BW tweeter and the latter an ultra wide BW midbass, so it will come down to which polar response/BW/distortion levels best meets your design goals.

http://www.adireaudio.com/Home/ExtremisMidwoofers.htm
http://www.creativesound.ca/details.php?model=EXTREMIS6

FWIW, I prefer the former combined with a suitable LF system to keep various distortions low, but many folks (you included, based on your stated preferences) seem to find audible harmonic distortion quite pleasing, so the latter no doubt will continue to be very popular as a ~ 'one size fits all' option, and you can always add a super tweeter to it to fill out the extreme topend. For a modest system though, the latter is a better choice IMO if the critical mids are clean since it will safely go a full octave lower.

GM