I've done each, just once - loved them both for different reasons! Both were good calorie burners (slightly less than Insanity and IMAX 2, but I recovered faster from both of Mindy's).
The Best of Bosu is what it sounds like - a great survey of favorite moves by a variety of instructors, adapted by Mindy. That makes it 'feel' interval style, and there's such a variety of techniques that the entire thing is very entertaining.
Workout! feels like "just us two people" having fun working out together. It is also interval style, structured with a few introductory moves and then a blast afterwards, in series throughout the program. I did this one on a day when I was feeling rather blah and it was exactly what I needed - Mindy's funny in a best-female-friend kind of way, and she works out in a style I can only think of as joyfully and intensely lighthearted. While you don't have to have a bosu or step to do it, you can use both if you choose.
I'm planning to save Workout! for days when I need a change to lift my spirits and use Best of Bosu for any interval day when I want to do bosu work.