I own the Guarneri but I would have to say I have been impressed with all their speakers and from my experience the Extrema is their best speaker (have not heard the Stradivarus.
I spent a afternoon(whilst on holiday in Italy!) at SF's agent in Bergamo who were very gracious and eager to let me hear the various ranges.
Here were our conclusions
1st: Extrema but only with a very good SS amp that could control them. Amazing full range sound including a bass quality to die for when we listened with Spectral amps (which mated better than Levinson).
2nd: Guarneri and Amati. Love them both, especially with strings and vocals. The Amati of course went deeper. Great with both the Spectral and Sonus Faber.
3rd: Cremona had a great overall sound, lighter than Guarneri or Extrema and worked well with all the amps we tried (Audio Analogie, Sonus Faber, Spectral).