I think they're balanced just fine.
Greatsword is the choice for consistancy. It's difficult to get that lucky 11 or 12, but equally difficult to get that unlucky 2 or 3 (and you'll never get the truly unlucky 1).
Greataxe is the choice for luck. Chance defies averages sometimes. Back in 1E when rolling up the hp for a high level Barbarian I rolled nothing lower than a 10 on a handfull of d12s. The same could happen when using the axe.