Bigger, more technical, better movement. Better passer. I like Ba but I don't think he's good enough to spearhead a top team. Llorente is used to creative football and could adapt to any role. He's deceptively quick too for someone his size. They've downgraded him on Fifa for some reason but he's got pretty decent pace in reality.
Basically, Llorente is a £25-30 million striker under usual circumstances. Ba is the guy you might take a punt on if he's available and you need a couple of goals.