I wouldn't use either on heather! I cut them back with shears a little after they've flowered, and then in the early spring to keep them compact. I think it might be all too easy to cut into the old wood, which heathers don't appreciate if you ran a shearer over your plants. I definitely wouldn't use a hedge trimmer on them!

As to the difference, I think you may be referring to a fully fledged purpose built power tool meant for close hedge trimming quickly and easily over a large area, where you probably have to sweep up the trimmings. Isn't a shrub shearer a smaller tool that collects the trimmings? If people only have a few feet to cut, or small shrubs to keep tidy, that's a possible boon for them..