<p>I'm getting into more of a TDD workflow, and have a mix of MVC and asp.net Web Forms apps.</p>

<p>MOQ is recommended for MVC.</p>

<p>I've used Rhino for Web Forms.</p>

<p>Does anyone have a best practice for having 1 framework mock for both?</p>

I would just go ahead and use my favourite framework for both. I don't think there's any reason that I would choose one framework for web forms and another for MVC. A far bigger problem is how I would unit test my web forms pages at all, since it's notoriously hard to seperate the page from the rest of the HttpRequest stack.
My favourite is Moq. I've also used TypeMock. It costs money, but it's really powerful - it lets you mock concrete classes and constructors, so you could potentially mock things like HttpContext or HttpRequest.