Either way this seems like a very clunky solution for arbitrary markup in shortcode.
If only one of attributes is more bulky and includes markup I would consider making shortcode enclosing:
[infobox src="http://www.google.com" title="Google"]
Some description - see more: <a href="http://www.google.com">More here</a>
[/infobox]
You might be to the point when you’ll need to built custom UI or it will just be too error prone for users.