I use Hinoka, Lissa, Eliwood, and Julia.
Eliwood is the only 4 star, and none of them are level 40 yet. I used Nino for the longest time before summoning Julia.
TBQH, having used Julia for so long, Nino is probably better. Julia is a good tank breaker and has a terrific weapon against Dragons but Nino's speed gives her a better damage per turn against neutral targets and they're about equally fragile. That, plus NIno's weapon has a much more useful skill IMO. The only conceivable reason I could see using Julia is her ability to heal aura people, something that's probably going to get you killed if they have AoE.
Though one of the two likely requires 20 000 feathers to get to 5 star.
My own team right now has changed a lot.
Reinhardt (Lv 38, 5 star) - Mobile cannon. Really powerful but goddamn he's slow. Anything he's attacking dies if his special goes off and has a Brave tome which is nice...but he takes insane amounts of babying if it isn't a 1 round kill.
Julia (Lv 40, 5 star) - Role hasn't changed. Glass cannon and anti-Dragon specialist. As mentioned above, kind of wish I would be lucky enough to get Nino.
Tiki, Young (Lv 39, 5 star) - Tank. Tank, tank, tank. Tank. Also breathes fire.
Lissa (Lv 28, 4 star) - Emergency "save me" button. Low upkeep on her with Renewal and Rehabilitate is a fantastic healing skill which can turn around an issue right away. Gravity is a nice toy if you need it.