Well it depends on your world and what contracts are offered and what talent is available. If you feel you have to max contract either of these, Myette is a perennial 40-40 player who can play every day while Arias has to rest and only has the HR potential.
If you let them walk you get the extra draft picks and can spend on an IFA, but will your picks or the IFA be guaranteed to be as good as either of these guys an how long will it take for them to contribute to your ML team?
It all depends on your window, your minors and a what your world is like realy...