If you're asking who was the most helpful, I'd have to go with Navi. She gives you more tips and is a better guide, BUT she can be a little over the top. She is pretty much Captain Obvious. This is not necessarily a terrible thing, but it often becomes annoying listening to her rants about how a door is locked. Tatl often said that she "Didn't know everything!" whenever you wanted advice or strategies though, so she wasn't any better. I usually don't use my fairies anyways.
If you're asking who is a better character, I would definitely say Tatl. Navi seems to be nothing more than a guide. She has about the same amount of personality as a stick and is OoT's equivalent to Pokemon's Pokedex. She doesn't really add anything to the game other than hints. Tatl, on the other hand, was a dynamic character with a more developed personality. She can often be rude and stubborn, but sometimes she is more forgiving. She changes over time.
So, in summary.. Navi is a better guide and Tatl is a better character.