One of the main differences between the Muscle SpikerBox Pro and the Muscle SpikerShield is the number of channels that it supports. 

With Pro, you can record two sets of muscles or from two people simultaneously, but also the variety of extension devices that it supports (Reaction Timer, Reflex Hammer, and Game Controller) compared to Muscle Spikershield. 

Muscle SpikerShield is Arduino-based and supports only 1 channel. Because it is connected with Arduino Uno, there are leftover analog and digital pins that could be programmed to interfere with other devices.

Muscle SpikerShield is a kit that is the brain behind The Claw and is our development board which allows students to begin to create their own muscle controlled human-machine-interfaces, as seen here in these student projects: Prosthetic Arm, Leg - And for schools who work with VEX Robotics, this is a great student example of a VEX Neuro-Claw
And a more recent example: Low-Cost DIY Prosthetic - Twitter Example: Hand in Action - New and Improved Hand! Example Science Fair Project: Detecting and Recording Muscle Cramps - 1st place!

It is also the brain behind Human-Human Interface but needs an additional teammate to pull it off