Wilson Sasha 1 vs Sasha 2.... What songs best convey tweeter differences?

These come to mind for treble tests.  Specific timing of when in the song, no idea.

Might try:

David Byrne, Grown Backwards (album), Lazy (song)

John Coltrane, Giant Steps (album), Spiral (song)

George Winston, Winter Into Spring (album), January Stars (song)

Lazy is going to push limits and you should hear either harsh or not.  I use this song for every new listening equipment, it pushes things a wee bit, through all the frequencies.  On Spiral you will notice distinct and subtle differences in the attack on the hi-hat.  On January Stars just play from the beginning and see which better compliments the piano treble. 

Good luck, happy listening.

I’v read the titanium "can be hot", "tiresome", "shrilly". The soft dome to be "better", "tamed" by comparison.

Above mentioned is a very simplistic way to look at a tweeter. They can also do the opposite depending on how the speaker got designed (as a whole).

The edges of all the transients get put in place for your perception by the tweeter. Listen for those. Try this... unique track (i.e., not your typical mindnumbing audiophile track that got played at the shows) It could let you know if the Sasha is worth your while or not before you open the wallet.

Baaba Maal - Gilli Men

https://www.youtube.com/watch?v=J5IRSe2h8CY

Look at a speaker as a whole. Think about how you will integrate them into your room. Speakers can sound hot, tiresome and shrill for many reasons and tweeter is just one potential culprit. Inadequate amplification that can’t get the bass right, room acoustics with untreated walls and floors and associated equipment are all going to be more influential than the material the tweeter is made from, at least in the case of Wilson speakers.
Don’t overthink it when it comes to the tweeter  

For recordings to use, try Diana Krall “Peel Me a Grape” - good test to see how sibilant the speaker is. It’s also a good track to test the bass.
Keb Mo “I Was Wrong” is a busy track that’s recorded extremely well and is good to test soundstage and how realistic the cymbals sound.