If you are thinking about those situations where there's no mousepad-sized stable surface for a mouse, well, in those cases I think a mouse is better than the a trackpad. I consider the most extreme scenario to be when the only surface you have is your thigh and I prefer a mouse for that. It can actually work with as little as an inch and it's firmly held by your hand, while a trackpad really needs to be more stable or you'll flip it when you push the wrong part.
Have you considered something like the Logitech Anywhere? I'm not the greatest fan of the Magic Mouse (and, no, it's not because of that port, I'm the only person in the world to think it's a good design choice) and in my experience it only works acceptably on a perfect plane and the right surface. I think the only good thing for you about the Magic Mouse would be the horizontal scroll if you really need it. It also has a different charging port from your iPad.