MBEx is probably the most themable of the three, and some amazing mods have been made for it. IF it works well on your phone, it's also the better deal pricewise over MCP--but for me, it delayed resprings to an unbearable level and caused safe mode crashes any time I had to respring after a Cydia install. The dev is apparently completely uninterested in fixing the issue, which is a fairly common one, and he does not respond to email. For a lot of people, it's a must have because so many themes are out there for it; for me, it was a waste of $2 that made my phone more frustrating.
Music Controls Pro is the least themable of the batch and the most expensive--but it's the most versatile and useful. You CAN change the background of the status bar drop down by theming your normal folder background image and toggling "Old background" in settings (under Task Controls). I'm still working out how to theme other things, like the buttons, did manage to theme the "no album art" image (it doesn't use any default you set). No one has made a theme for it, period--I spent a lot of time looking--and the dev isn't helpful on that score, though he provides pretty good support in other areas from what I've seen. But it works with Pandora, Tune In Radio, and a bunch of others. It's rock solid reliable, doesn't cause issues with other things on my phone, and it's full of useful features. I really hesitated over it because of the price, but I used the free trial--now I use it daily and wouldn't be without it.
So really, it's a matter of priorities. If you're OK with potentially tossing out $2 and you really want to play with some fabulous theme options, go with MBEx. After all, it's only $2 if it doesn't work for you. But if you use Pandora, etc. a lot, try the free trial of MCP first.
