Giving it a little thought, unless you've other plans, it seems that the unity-2d session should show Ubuntu-2d on the panel. It maintains continuity from the login & lets users know that they're on what they choose.
Or if they choose Ubuntu then it's an immediate sign that something has gone wrong.
There is also a surprising number of requests from users as to how to tell if they're on the 3d or 2d session. There's no reason not to make this a bit more upfront.