I think Big Al's upside was considered a little higher at the time, but his lack of development on the defensive end means he never quite reached his ceiling.
You have to consider Sully's green light shooting 3PTs as a contributing factor on offense. Take those away and he's closer to 48% for his career, while Big Al has been regularly in the 49-50% range.
All in all, Big Al was a slightly better scorer, slight better rebounder, better shot blocker, taller...
So in short, I think yes, Big Al was the better prospect *at the time*, but Sully does have the potential at least to have an even better career.