I’m sure Cassie’s got skills but I’ve read Amethyst’s entire original series while I still haven’t read any wonder woman comics with cassie so I’m gonna go with who I know. Cassie may have been trained by the Amazons (or maybe she hasn’t, idk) but Amethyst has been trained by Citrina (and Doctor Fate a bit) in magic and Lord Garnet in physical combat, and I reckon Lord Garnet could even make someone like Wonder Woman sweat when it comes to martial skill. Also Amethyst is a Lord of Order. Just like Doctor Fate. I heard she even helped The Specter hold the universe together! (still haven’t read Crisis)
In terms of power and skill I think she would stomp Cassie.
Unless Flashpoint or Rebirth changed something while I wasn’t looking, Amethyst has raw magical firepower that I don’t think Cassie could adequately counter.
Apparently, whenever Earth suffers a continuity changing event like Crisis or Convergence, Gemworld is similarly affected, though not to so great a degree in that it’s denizens can remember how things were and observe the changes. By their count, this has happened SEVEN times since the original Amethyst series.