Green mage choice

I say wait until you find a +spd/-def Nino. its her optimal iv set and it even lets her (with Fury) bait and kill Reinhardt so long as he isnt using Moonbow and Quickened Pulse

Julia is a more standalone mage, nino isnt anywhere near her maximun potential without buffs.

It looks like Julia is a better fit for your team even though Nino has better IVs. Nino is one of the best nukes in the game but she needs team support.

Julia is much more a defensive mage, while Nino is an offensive mage. Both are really good, but whichever you use depends on your team composition. (Other than the team you listed). But for that team I'd say Julia because Reinhardt is your offensive monster, a role which you don't really need two mages on in team for.

Both are good, although I think I prefer Nino, since it will be easier for her to reach desperation (assuming you have fury build)
Just make sure anyone on the team have hone or fortify for buffing her.

Julia's IVs are good too, I think.