I prefer crystalline. I imagine they'd have one of the strongest hulls in the SFC universe, unsure about SFB though. I mean strong enough to rival the Gorn. In KA, the crystalline hull they use were stronger than that of Gorn vessels. And being such a unique and exotic, yet semi-aggressive (*shrug* I dunno. I like the Tholians but don't have a whole bunch of knowledge on them) species, I doubt they'd use what everyone else uses.
I like the design Atrahasis, I say go for it! But there is a need for some recoloring. Even though it's kinda cool, at the same time it just doesen't look right. I can't explain it. Mabye it's too 60s-ish or something. It needs a little modern look. Mabye a blue-ish or neon-purple color while keeping a crystal feel to the hull.
One of my favorite non-canon designs was KA's Amethest-class Tholian BCs, I like it's design. I also love Brezgonne's "TNG-ish" Tholians too. They just need to venture away a bit from the standard Tholian look from the copper hull, they'd be a bit more interesting then.
BTW, since I'm getting back into the SFC2/SFC: OP gaming, I do have one question: What patches do what? I see more and more newer SFC models are included in the patch, such as Andromedan ships. Anything else happening?