Magnetic if you had to pick between those two only. You can buy MND+2 or get MND+3 Mamool Ja Earring.
Aside from that, the real decision is between Ethereal or Hollow IMO. Level a tank or melee, and you'd be inclined to agree. You want the more unique beneficial stats from AN earring. MND+2 is AHable, and there are better nuke earrings for BLMs, so Conserve MP on Magnetic looks more appealing. But having built a pretty decent Conserve MP set on my RDM and having owned Magnetic for a year, I can say that Magnetic Earring doesn't match up to the value in owning Ethereal or Hollow.
Question regarding MNK without Shura Togi / Usukane Haramaki / Enkidu Harness in foreseeable future:
Antares Harness or SH +1? For average meripo with Pizza, TP and probably Asuran piece. ACP body is used up already.
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