IMO, Harden is as good as LMA right now, and will likely get better, considering his age. I'd do the LMA for Harden thing if it was proposed and could be made to work. As the main focus of the offense, Harden probably isn't quite the iso player Roy was, but he is a better shooter. He's also got a long time left in his career to improve. I would package both picks and a salary-matching player for Harden; he's that good... Never gonna happen, tho.
I would not trade LaMarcus for Harden, you really need two stars to contend for a title and I think switching those two would be more of a lateral move. I would trade all our other assets to make it happen; Wesley (a cheaper replacement), #6, #11, Elliot, Babbitt, Freeland and Clavier. But as I said earlier I don't think OKC would be at all interested yet though, they can amnesty Perkins or likely trade him for an expiring contract. I think they'd go with all minimum salary auxiliary players before they break up Durant/Westbrook/Harden/Ibaka.