ENFAMIL OR SIMILAC?!

Honestly, I'd say it really depends on your baby's tummy rather than one being universally "better", some mums swear by Similac and say their babies do really well on it, while others found Enfamil gentler on digestion. Since your little one is already on S-26 and suddenly constipated, it might be worth trying Enfamil to see if a formula change helps, but I'd also check if she's getting enough water between feeds (especially since it's warm here) and maybe chat with your PD about it, just to rule out anything else. My neighbour actually switched her baby from one brand to another when she had the same issue, and it made a difference within days, though every baby's different. The good news is constipation from formula is usually quite fixable once you find what works for her tummy. Definitely worth getting your doctor's input though, especially since she's half breastfed and half formula-fed, they might have other suggestions too!