beryllium vs diamond

In theory, the "diamond" ceramic tweet "should" have better extension & resolution than the be. Sound propagation is extremely fast & distortion characteristics are, allegedly, benign. The accuton diamond has a very low res frequency @ ~900Hz, which is great. As to whether these qualities make a significant difference to the music/audio phile in the 20-100kHz range, is another question altogether -- it's basically a matter of implementation in a speaker system

Compatibility-wise, I would prefer the ceramic tweet to mate with a traditional wide-range low-mid/bass driver (say a Supravox 400exc), for an exquisite two-way speaker -- on paper. But, at $~6k/pair for the tweets and ~2k for the other drivers, it's a very expensive experiment!

Also, be is toxic as rp1 notes (although a Grande Utopia seems hardly a domestic hazard!).

Has today's dome tweeter better resolving power than the venerable electrostat?

IMO traditional stats' strength lay in the "mids" range (i.e. s/where within the critical range for our ears ~150-10kHz). Modern tweeters are usually hi-passed at 2-3kHz reaching supersonic extension thereafter (~100kHz for the Thiel& partner -- accuton -- "diamond")

As to which is better, be or ceramic... difficult to say, it depends on what you're looking for: both are low mass hi rigidity matls (good), the on/off axis resolution is helped (the dream), both (can) store a lot of energy (not so good), both are expensive (very bad:)).

What's the application you're looking at re these tweets?