I chose Similac advanced concentrate that you mix with water. I chose it because of the price and my 3 month old loves it. No gas or spit up problems. And I get coupons in the mail for it. If it give her gas or she spits up or has diarrhea then I would change it, if not any of those then I say it agrees with her. Hope this helps! :)
I reasearched formulas and majority are pretty much all the same. If there where a formula that was unique by law they'd all have to contain the specific ingrident that makes it unique. I chose apitmil 1st infant as it's the closet to breastmilk substitute and doesn't contain any fake sugars or salts, it's also independently owned you can also buy them in ready made bottles which can be given at room temperature and are ready sterile threw the manufacturing process. I wouldn't recommend any stay down formula as these are made with chilled water which won't kill any bacterica in the fomulas as no powder formulas are sterile.