Musically, marching marimbas don't differ much from their concert equivalents. The marimba is a cousin of the xylophone and other "keyboard-style" percussion instruments, featuring a fairly large layout of keys atop tubular bells that amplify the sound. The biggest thing that sets marimbas apart from other melodic percussion is their use of resonators to create their characteristic tone - and what sets marching marimbas apart from ordinary ones is that they're designed and built to handle the rigors of being rolled out onto the field. Musser's M501 Coliseum Grand 5 Octave Kelon Marimba, for example, sits on an all-terrain frame with big, knobby tires that easily roll over the turf, even if it's soggy from a recent rainstorm.
Aside from the marimba itself, don't forget to take a look at all the accessories and supplies you'll need to transport, maintain and play it. Cases, for instance, will go a long way to getting the instrument from place to place in one piece - the Adams Academy Series Marimba Soft Case with Wheels is a great choice for Pearl's AMPD30 series, and there are equally good options for any marimba you might happen to choose. Add on a good collection of mallets and cleaning essentials, and you'll be well-prepared to keep those marching marimbas looking and sounding their best for years to come.