I love the Shadow's size and ease of use. Having the input and output jacks on either end really helps over the Hornet/Tomahawk.
The Shadow is an IEM amp only, whereas the Mustang has further options, and only a bit larger in size. I keep the Shadow for true portability, and only use IEMs when not deskbound.
If you are still in the stage of trying different headphones all the time with your portable setup, I would stick with the Mustang.
I agree that the SR-71 probably has the best sound of the lot of his amps, however it runs on 2x 9volt batteries, and is biggish for portable use. If you are going to use hard to drive headphones on-the-run, it might be your best choice though, and find some good rechargables.