It really depends on what ships you have. Quite often for me, merL is the more scarce resouce because a lot of fleet ships (not zenith) use way more metal. The reason that turrets and starships both lean on crystal is to make them opposite of most of th. Original games fleet ships. Thus there is some subtle encouragement to use a variety of ships. With zenth, a lot of even their fleet ships do lean on crystal more, too, which does make that more of a commodity -- this seemed fitting for them, and having a zenith specific shortage was something that I felt would lead to differing strategies as far as which planets to take, given that there are uneven resources of each type on the planets, too. Most likely with the next expansion I will emphasize metal more.
Interestingly, though, I generally am short on metal even now -- maybe because I build few starships, though I do build huge forests of crystal. So I think some of it depends on both ship mix and playstle, and that then impacts which planets will be most attractive for you to capture.
