Spring Sharena IV's

If you're keeping her vanilla build then go for +res, but if you want to use an offensive or blade tome build, then use +spd

+Res -Def turns her into a potent mage tank

I'd actually say +spd either way. The situation is that mages are extremely fast these days (Ishtar, Aura+eff on Linde, Nino+10 if you're in the 716ish scoring range) so even enemy phase builds are at risk without 42+ spd. And if you want to attack, of course +spd.

The only way I recommend +res is if you want to ploy with the prf and no fury. That also makes sense to me, especially if spd ploy is involved. But you could go +spd, fury, and still ploy, so...