There's no difference. It's all marketing.
Think about it - it would be completely inethical to make one (expensive) milk somehow "the best" - with more of whatever it is that ff manufacturers deem that babies need - and to make the cheaper milks "less good", and therefore deprive the babies of poorer mums of better quality milk.
sleepyhead has it exactly right - Aptamil is marketed at women who feel guilty about ff. Hence the price tag and more "scientific" name and packaging. Women who intend to ff from birth and feel confident about their choice generally choose Cow and Gate or SMA - these are "heritage" brands, they've been around a long time and the chances are these mums' own mums used them, so they're familiar brands.
But if the packaging was plain and the powder was analysed, it's the same stuff. If you intend to bf, don't buy any. If you need it in the future, buy whichever one you can get hold of easiest.
And there's no reason to stick to one either. Again, manufacturers make us think there is, because they know they've got a captive audience - once they've got you using formula, they know that for 6 months, that's all your baby will eat. So it pays well to make you think that swapping brands will "unsettle" your baby. There's actually no evidence for this at all and in fact swapping may be beneficial, in that different milks may have a very slightly different taste (even though the ingredients are the same) and therefore ff babies can have an approximation of the bf experience (breastmilk changes taste from feed to feed).