Putting items next to each other
My opinion is that for simple tasks like in your situation, you should not complicate your XAML. Wrapping a StackPanel around two TextBlock-elements is still pretty clean.

Extracting the code and wrapping it in your own control where you would set the Label and Content surely is an option. But if this only to be done once, I think you're overdoing it. Only when you have to place the same code there a lot of times, your XAML will be cleaner using your own control.