Srry to post in an old topic.
Actually I'm not to sure on this, The second war was with the Sannin vs Hanzo excreta. The first Shinobi war was with Team Tobirama vs GinKin.
Hashirama actually dies before that lol in one of the "wars" after the founding but "before" the First Shinobi war, talk about pure fail on his part.
So the gulf in experiences goes doubly for Hiruzen, he managed to outlive them all by a wide margin and fight far larger and higher stakes battles than any of them. The clan wars were by comparison in scale a set of border skirmishes. Clan vs Clan does not compare to Village vs Village. Honestly he makes all the Hokages feel like fails to me, they may be more powerful but they certainly weren't smarter lol.