It annoys me when people say Seph was the best mercenary in the world. Well yes he was... everyone else sucked when he was around. His attack when a SOLDIER was in the 3,000s, that would be about 3% of what an attack from Auron can do.

Sephiroth had no power other than Jenova, he was used from before birth right up until just before he got his ass whipped by Cloud (with plenty of occassions where he gets thrashed before then).

Auron didn't care about Spira enough to act as a sensible leader, he was there to teach two of the most useless people in the world how to be grown ups, which he did without once smashing either of them round the head.

Auron cared about his word, and drunk whilst keeping a cool head in battle. Sephiroth either got used as a puppet or whined about his mother.