Different babies will prefer different things, but until you have a reason to believe yours doesn't get on with one, they really are all the same, so take your pick. SMA tends to be more constipating for a lot of babies, so I'd avoid that to start unless you run out of other options. Aptamil and Cow and Gate are the same, made by the same manufacturer, same ingredients, etc. Aptamil is just more expensive because they spend a lot more on marketing. We used Cow and Gate as my daughter was happy with it and I couldn't see spending the extra 3 quid every time for Aptamil. Hipp Organic is another option if your LO doesn't get on with any of the others, and many find it's more gentle that other brands, but it caused my daughter to develop reflux and horrible projectile vomiting, which is how we switched back to Cow and Gate.
That said, if you are planning to BF, plan to BF. I wouldn't worry about stocking up on formula in advance. My daughter had all sorts of latch issues and struggled to gain weight and we had a horrible time with BF. But formula wasn't an emergency sort of thing. I didn't have any around the house and I didn't need it until I decided to switch. When we did decide to use formula (basically, I got mastitis so badly I was pretty much unconscious with a fever and my body wouldn't let me pump enough milk to feed her), which was around 10 weeks, we still had time to run to the store and get some formula. If you want to BF, the best way to ensure it's successful is to plan to BF. If a day comes that you do decide you want to use formula, you can always go get some then.
