I gave NAN in baby's early days because I heard from other mummies that the taste is the most similar to breastmilk. It was okay, my baby never got constipated and grew well. I switched to Lactogen to be more economical after baby turned 1 yo. I guess every baby reacts differently, no harm trying both to see which one works better for your baby.
My baby is drinking NAN supreme pro HA. Switched from S26 to NAN because S26 is too heaty. Nan supreme pro HA is recommended by my PD as it is similar to breastmilk, easier to digest. She recommended similac HA too. You can try and error to see which formula milk is more suitable for your baby.
IMO, if your infant has no adverse effects with drinking similac, ie no upset stomach, diarrhoea/constipation, rashes etc, just continue with it unless theres another reason to switch formulas.
I was told if baby is doing fine best not to switch the brand of formula. If baby having issues with the formula being used then can consider to change the brand.