I know this is quite old, but I'll give my answer for the benefit of anyone else arriving here.
Full disclosure, I worked for Appcelerator from 2012-2019.
In short, they're both quite similar.
- You write a specific flavor of JavaScript that includes custom objects
- Your JavaScript code gets compressed, minified, etc. so it becomes the input to an interpreter that gets shipped with your app
- When your app runs, it launches the interpreter which starts executing your code
- At this point, JavaScript is being run in "native land", so the interpreter can act as a bridge between your JavaScript code and the native SDK
- The result is a native app with access to features of the underlying platform
A "native app" shouldn't simply be defined as an app that runs natively, because that would make a PhoneGap app a native app.
A "native app" should be defined as an app that runs and behaves natively. That means, an app that abides by the native OS' ui/ux guidelines.
MAIN DIFFERENCES?
I'd have to say that the main difference is its internal design. Appcelerator started in 2007, when "mobile" was very young.
React Native was launched in 2015.
In 2015 mobile was already in full swing, and mobile OS' were pretty mature, so I infer that React Native's design benefits from all lessons learned by Appcelerator (and others).
As for which one is better, I'd say React, because Titanium is now dead.