Ion is lighter than the Impulse, smaller than the Impulse, can cycle just as fast (if not faster). Is more customisable than the Impulse, takes the same barrels as the Impulse so you can take any nice barrels like a freak kit over. Parts are more readily available for an Ion than an Impulse also. I think an Ion is also cheaper than an Impulse.
Although an Impulse with a good bolt, hammer, hammer cap, and maybe even an LPR will sure increase efficiency.