Your guess is as good as mine, but I'd venture to say Marte is up in the next week, while Myers gets the call no later than the trade deadline. I see Myers being more productive regardless, unless you overvalue speed. I do think Marte will claim a spot in the batting order that's more conducive to putting up counting numbers, though -- somewhere in the top three, probably, whereas Myers should hit sixth or seventh, and no higher than fifth.
Then again, Marte conceivably could be part of a Justin Upton trade, in which case he might not see much of the majors this year.
If I'm taking a flier on one prospect, it's Myers. (And I did.)