I would probably say that typing is ultimately the deciding factor. While there are obviously some exceptions to the rule, like Blissey, there's a reason why they are exceptions; they are exceptional. Not many (no) Pokemon have the raw defensive stats that Blissey have, and so it doesn't matter so much about her typing. However, Pokemon who are not so blessed, like Scizor and Dialga (although Dialga's stats are great), rely on their wonderful defensive typing. Even though Scizor's Special Defense is nowhere near high, it has little problems switching in on some powerful Dragon-type attacks due simply to its Steel-typing. If Scizor wasn't a Steel-type, I doubt it would ever be used in Ubers at all (especially seeing as Bullet Punch would lack STAB).
But yeah, obviously when there are exceptions with monstrous defensive stats like Giratina, Blissey, and Lugia, then stats are more important.