[quote=Rtron]How is choosing one better than being able to choose both?
[/quote]

It's probably not. But federal voting leads the focus away from local politics. I'm thinking that if your federal voice was the same as your local voice, then you'd use your local voice more (which is fitting, because local elections take place much more often than federal ones), leading to a more involved and better-represented populace (and better governance on local levels, too), while generally de-centralizing the power structure.

edit: clarity