The Mariners should keep Omar because he is a first-string with pop who hits for higher than Murphy. And it makes sense to keep Austin Nola over Murphy since he more fits into this rebuild, being just a rookie with more seasons ahead of him. Not to mention Nola rocks a .308 average through his first 16 contests.
The only argument in favor of Murphy over Omar is the defense. Despite playing almost half as many games as Omar, Murphy has three fewer errors, a better CS percentage, and a better fielding percentage overall by two points.