Well, my guess is that Devices are items that you hold, when you fire them, they do a special effect. They could also be a attachments to weapons.
Mutations are going to do stuff to your hero. My guess is that it is going to make them run faster, better aim, etc..
Of coruse, we will never know until they are released!
Thats just it, the in-site store has examples, and judging by the -very- minute effect of most of them (besides the "Majors", which might be active item uses, or might just be limited due to thier power), they seem to all be totally passive boosts, like the Products in Super Monday Night Combat. You bring them in with you at the start of a match, and they give you a tiny boost in something you just need that -slight- nudge in. Say, you're using a weapon with a hell of a slow reload, so you equip some devices to make your reload more manageable.