baby milks, what are the differences??

hi this is my second baby and i used SMA with my fist which was fine...i am hoping to try breastfeeding with this one but dont see me doing it for more than a few weeks/months (but have been told this still benefits them) ao anyway, i just wondered what is the difference between baby formulas, as i would prob just use SMA again unless anything else was better...i was clueless last time adn not much better this time, and just picked SMA cause it seemed the most available one and easy to find.