it's a tough one, last season it would have been an easy choice ... I think demar is a little better, but butler is a better building piece. Salary wise, soon butler will have to be paid, so long term it's not that big of an advantage.
for me, demar has the same issue as bosh, what he does best, he doesn't do it well enough. And you absolutely have to pair him with a defensive specialist on the wings, you don't want demar defending durant or lebron or george in the playoffs. Butler you can plug on any team and will fit like a glove ...