Carmello by a mile...
Melo is arguably the greatest pure scorer in the entire league. Like Pierce in his Prime, Melo can score in any manner of ways. In the post, off the dribble, from three, from midrange, from three, on impossible fadeaway jumpers, getting to the foul line...he has no weakness in his offensive game, and he's statistically one of the greatest clutch scorers of this generation.
Melo is also more versatile. He can play SF or PF, and could probably even play SG at a squeeze.
For his hes an elite rebounder at his position and a very underrated passer.
Defensively hes actually pretty competent 1-on-1 but isnt a good team defender.
Love is nowhere near as versatile as a scorer and he can only play one position - PF. Even at that position he is physically disadvantaged most of the time. He shoots similar percentages from the field despite being a PF (wgich by position should shoot me efficiently thana Sf...but doesnt.
Melo can be a #1 option and can carry a team to the playoffs - we know this because he has many times. In fact rarely have his teams NOT made the playoffs. Love on the other hand has never led his team to a playoff series, and there's no signs to show that his numbers arent boats by being a good player on a bad team. Melo has maintained his scoring numbers consistently on every team he's played on, no matter good good or bad the supporting cast has been.
Melo over Love for me is a no brainer.
Oh as for the prima Donna argument...how is love any better? He's been on record multiple times complaining about his team and talking about going elsewhere, and now that his contact is coming up he's already demanding a trade... Even though he's a young player who's never led his team anywhere.