Harvin, at his best, is a better pure kick returner. His athleticism is other-worldly. I remember a Baseball Prospectus tidbit on Rafael Soriano when he was at the top of his game. They called his fastball 'Lovecraftian.' That'll do for Harvin's innate athleticism, too.
Harvin has rare top-end speed, rare agility/elusiveness (even in tight spaces), vision, understands blocking schemes and sets up his blocks accordingly, neigh Golden Tate-esque gyroscopic balance, and toughness/bravery.
Lockett has good top-end speed, excellent agility/elusiveness, intelligence (football and otherwise), vision, understands blocking schemes and sets up his blocks accordingly, work ethic, and clutchiness (he steps up against top-end competition. Seriously, it's in his scouting report.).
Harvin's rare athleticism is what separates the two. In the return game.
The difference in athletic ability is small enough that the difference in skill becomes the deciding factor. Or is it the other way around?
Harvin has hands that are 1 inch larger, arms that are 1 inch longer, 10 extra pounds of muscle, and innate brain/body control that is at the pinnicle of human ability.
Lockett likes to block. Harvin dogs plays not going his way.
Lockett is a super-polished route-runner. Harvin depends on his freakish physical ability to make the spectacular play (and comes through).
The durability concerns that dog the both of them have blossomed for Harvin but haven't yet shown up consistently for Lockett.
Exec Sum - Harvin, at his best for one play, is far superior. Lockett, consistently, is preferable to have on one's team.