As far as I know every webbrowser that is released for iOS is not allowed to run its own browser engine. For example (by my knowledge) Chrome for iOS uses Apple’s browser engine.
The difference between browser engines are support for standards, being or not OS, implementation of the web rendering… Between WebKit and Guecko are a lot of differences, yet both of them shows the web as it should be. There’s not more than that kind of stuff.
I don’t know anything else than that, sorry if it’s not much.