Well, I figure at some point there are no more levels - on a 1-5 star scale even though you might want to give something 7 stars, the review will still say 5.
Finally, the newer conclusion that I've come to based on the past few pages:
The Sharingan is better at one-on-one fighting and quick skill enhancement.
The Byakugan is better at multiple-opponent taijutsu fighting (probably ninjutsu; I'm not too sure) , spying, scouting, detecting traps, and pretty much every other occasion that doesn't require direct combat.