Actually, Blue, I was wrong: I shouldn't be concerned with the racial critical hit die being a little lower. If a crit is scored, the sneak attack dice are doubled anyway.
Let's assume an 18 STR
Barbarian 5 Fighter 3 crit w/ greataxe: 3d12 + 4 (23.5 avg)
Barbarian 5 Rogue 3 crit w/ rapier...