The base build contains all the additional content; fixes, new clothes, additional weapons, additional homies, new dealerships, additional music for sale, additional garages for customizing, extra language strings, and probably more stuff I've forgotten.
The optional menu is only a small subset of the content that was changed instead of added to. It's mostly stuff that people complained about during development of the mod. i.e. someone didn't like the new spraytags? Add an option for it. etc.
The base patch is 1.2 Steam US and works fine with the UK version. It's really only there for dumb people who can't figure out how to verify integrity on Steam and have it replace the modded files with the correct ones. I should probably just remove it entirely.