Which Julia should I keep?

+Spd/-Def is I think your best bet. You can always give her death blow if you want her attack boosted. Taking away her res won't make her as good a counter to other mages like Spring Camilla or Sonya, or even blue mages like Delthea or Reinhardt.

+Atk

It depends on which one you like. I don’t think -res doesn’t hurt Julia’s magic tanking. But if you feel like it does then use +spd, -def.

It depends on which one you like. I don’t think -res doesn’t hurt Julia’s magic tanking. But if you feel like it does then use +spd, -def.

I'm using a +Atk-Res Julia as a check on blue mages and she does her job pretty well.
For better survival, go level up the +Spd/-Def Julia but in meta and missions,go for +Atk/-Res Julia.

I don't see how some more speed will help her with how low it is, I'd go for nuke, just sad for res bane but should still be ok, I have the same IV on my sanaki and she is more than ok

Neutral Julia is about a Fury away from oneshotting both Reinhardt and Camus. +Spd allows you to potentially be able to double slow (but not that slow) threats that you otherwise wouldn't without it, such as Hone Xander. You're also going to want to couple it with Desperation if you go for something like Darting Blow (surprisingly, not a bad choice for +Spd).

Long story short, +Spd versions of slow nukes such as Julia, Sanaki and Lilina are massively underrated. They tend to be able to oneshot enemies they're supposed to oneshot no matter what, so the +Spd either takes you away from getting doubled or even allows them to double.

+Spd/-Def

+SPD is a +4 increase that brings her base to 30. If you run fury on top of that she isn't going to be doubled nearly as often as she would at neutral speed. Her resistance will also remain untouched, which is good for her secondary function of dueling green mages.

Bottomline: +SPD will change the results of more matchups then +ATK.