- Callisto orbits a gas giant, Jupiter. Callista orbits a giant gasbag, Newt.
- Callisto is Jupiter's fourth satellite; Callista is Newt's third satellite.
- Callista made Newt convert to Catholicism; Callisto doesn't give a damn.
- While both are clearly ancient, Callista is not as heavily cratered as Callisto.
- Though it takes careful measurement to demonstrate this, Callista is slightly less spherical than Callisto.
- While neither shows any evidence of internal activity, Callista can appear to be active on the surface. The importance of this activity is unclear.
- There are many amazing and fascinating images of Callisto. Callista, not so much.
- Callisto's surface composition is overwhelmingly ice. Callista has significant amounts of clay, spackling compound, and pigmented proteinaceous material, in addition to ice.
- Callista gets upset when asked "Why so serious?" Callisto never gets upset.
- Finally, Callisto is often seen in the company of Jupiter's other satellites. Callista is never seen with Newt's other satellites.
So again, the confusion is perfectly understandable, even excusable. But these are the sorts of fine, nuanced observation that are necessary for making sense of a complicated, chaotic world.
Followup: oh yeah, of course there's that small matter of the terminal vowel. But I was focused more on the actual features, not so much on the fine details of spelling.
Followup II, Thurs. May 18: Peteykins at Princess Sparkle Pony beat me to this by nearly 2 months. I read that blog, so there is a strong possibility I unconsciously swiped this idea, forgetting I had seen it elsewhere. However, there is surprisingly little overlap between our points of distinction, so you should go read his post too.