Both are offensively inept which means what do you value more, overall defense or shot blocking defense. I think Asik is the much better overall defender where Sanders isn't as great on the pick and roll or stronger enough to deny his defender out of his original post position thereby making his low post longer and harder to put down.
Sanders can definitely block shots though.
Sanders will be very costly for years to come whereas Asik is only valued at $8.4 million against the cap and expires after next year.
Given the recent attitude and behavioral problems both exhibited, Asik quitting on his team, Sanders bar fight and so forth, I think having someone on a shorter contract in case the attitude manifests itself into something worse would be smarter.
So I go Asik.