First, I like Alexis and wasn't happy at the time we sell him but I understand why we did that. That said, I think Suarez is a better player and fits this system better than Alexis would. But I don't think it's fair to compare his goals to Alexis. Suarez is a CF and Alexis is a RW so it's understandable that Suarez score more. If anything we should compare Suarez's goals to Messi's when Alexis was here. But this doesn't make sense because we don't play the same system now and because well, Messi is Messi. You can't really compare anyone to him.
Maybe we can't compare Suarez and Sanchez on goals scored. But we certainly can do so in assists, and Suarez has him beat in that department hands down, and also in chances created. And yes, Suarez is a major upgrade on Sanchez, and there's no two ways about it.