The gold formula's have extra iron in them so they can cause constipation. The gold formula's also contain omega 3's (the heinz regular includes this also) neucleotides ( the heinz regular I'm pretty sure includes these too) The karicare gold includes prebiotics and the heinz and nan one's contain probiotics which is supposed to help with healthy gut bacteria. The s26 gold contains leutin which is supposed to help with the free radicals with the eyes. So it's basically if you want to pay extra for those little extra ingredients whether they are important to you. Honestly I am not a fan of s26 at all the gold is notorious for causing constipation and the formula is sticky and has a horrid smell. For me if you want to go gold the Heinz gold or Nan Ha gold are the best to try first. NAN HA is partially hydrolysed so the proteins are broken down so its easier for bubs little belly's to digest. It's all whey though where the sma is mainly casein so you would find bubs would be a lot hungrier sooner on something like the NAN HA.

BUB's don't need the gold formula's its only if you feel its necessary for those extra ingredients. I like the Heinz because it has the omega 3's in the regular one and neuclotides and it's a really reasonable price. I also like the bellamy's organic because of course its organic. I have found all my bubs are different though. My boys both settled better on different formula's. DD is predominantly breastfed but she has one bottle of formula which is the NAN HA and she has no problems with it. Unfortunately it can be a bit of trial and error. But always give formula at least a week to see how it goes. It can take a little while for little belly's to get used to them. HTH