Chriss is slightly better than Bender. I honestly don't think either player is good but if I had to pick one it would not be Bender but that's not because Chriss is much better.
However, Bender is worse at rebounding (especially defensive), worse at blocks, worse at opposing defensive scoring on his match ups and is not really an offensive force. The main thing he was supposed to bring to the court is spreading the floor. However, his vision so far has been limited, is not a great passer and isn't filling it up. The one main advantage has not materialized.
Honestly, the team could probably miss both players and not miss both players. The only reason either was drafted was based on potential. Both are still young so it comes down to who has the most potential. However, that potential is questionable at best.
If I absolutely had to pick one to keep it would be Chriss.