In what application are you speaking of here? Some units I find work well with one over the other, regardless of edition. Kiro's point is quite valid for a Blood Angel player whose Assault marines can take Meltas, but for a Space Marine player they're boned and can take neither. Only Flamers, Grav and Plasma Pistols for them. In my tactical squads from a BA perspective I play them with a Multi Melta and a Plasma Gun, since I find the Melta Gun to be a bit too short range for their application. In my Tactical squads for my Imperial Fist army, I take a Plasma gun and don't bother with a Heavy Weapon as it usually just isn't worth the points. I put those points into Devastator squads that benefit from the Tank Hunter portion of their Chapter Tactics. However, a Command or Sternguard squad in a Drop Pod kitted with Meltas is killer for vehicles. Combine a squad of them with a couple infantry killing Dreadnoughts in Drop Pods for a hell of a lot of fun.
I guess what I'm getting at here is that both guns are valuable and have their place, it really depends on what your given methods of war are and how you like to play the game.