Aang has homefield advantage, but unless you take away Eragon's 12 death spells, it's pretty unfair. I'd say Saphira is more maneuverable than Appa, but not more maneuverable than Aang on his glider. I know for a fact AS Aang wins. First fight is a bit more difficult to guage. I think though with his multitude of fighting skills and massive variety of attacks, he can wear down Eragon's shields and just work him over until he wins.