Of the three you mention though, I found SMA quite gloopy, and the powder was a bit 'squeaky' if that makes sense. Don't remember using Aptamil, and we used Cow and Gate Stage 1 through to about a year - you don't need to bother with Stage 2/toddler milk.
personally i think cow and gate but all of them are fine so its just a personal preference really. pick any that you like and see how your baby gets on with it and if it seems fine then just stick with it.
SMA gave baby constipation. Aptimil very popular so I assume it's good. We use Hipp Organic - on advice of a FF friend - I couldn't face researching them all after 2 weeks of nightmare trying to establish BF!
In the River Cottage baby cookbook, it says that all formulas are pretty much the same - they're all fine as they all have to meet the same standards to be sold in the UK.
cow and gate i would say, or hipp organic... i am going to express/ combination feed, rather than breast, as i have ME, and if i express and bottle feed, it will mean boyfriend can do it too, and i will get my much needed rest!
we used cow and gate with my first and that's what i'm using this time round i think cow and gate is the most popular choice but once you have chosen one you have to stick to it so i guess its hard to compare
Apparently C&G and Aptimil are more or less the same thing.
Aptimil is more expensive because it's marketed at women who bf and switch to formula and apparently it "reassures" them that it's expensive therefore best.
Sounds crazy, but when I was standing infront of the formula and had no idea, I did buy Aptimil and it was because it was most expensive and because it banged on about breastfeeding on the label (ds was mostly bf). And even know I knew it was all bollocks I still swallowed it hook, line and sinker.
Basically they're all the same thing tbh. Get the one that will be easiest for you to find at short notice (eg the one the all-night garage down the road stocks), by all means the cheapest one (it won't make it less good than the more expensive one - the basic ingredients are tightly regulated and the extra stuff is mainly for marketing purposes), and if it doesn't agree with your baby then it's fine to switch to another one.