I think that is a pretty easy choice. You have Gordon who I like more than Marte, then you even have Ian/Laroche as back up. There is no reason why you need 4 3B prospects, I'd prolly even deal away one of the others behind Gordon as well.
Kemp is a toolsy prospect that is really starting to get it, a potential 20/20 or more w/ good avg. As long as he doesn't have a major slump, he'd rank right up there w/ Marte after this year.