What I prefer most about an UI is the better sense of overview over the repository and it's branches.
I find, when compared to people I've worked with that prefer plain git, that I'm much quicker at finding certain changes or seeing what is in which branch with GitExtensions.
Developing in a Windows environment, I generally prefer Git Extensions.
It doesn't distract the eye with unneeded fancy and is very close to a command line git experience but still allows me a better visual sense of the repository and branches.