That's a bit of a tough one. I prefer Belt over Nishioka if you are straight up comparing the two, however Belt doesn't seem to be receiving consistent full-time duty in the big leagues. This might give the edge to Nishioka for this year.
For this year both are rather untrustworthy but I would say Belt long term and Belt as long as he stays in the majors. If another demotion looks to be happening than you might as well stand pat