Fierce Deity Link is much better than Dark Link. DL had a minimal role in the Water Temple, and was easily beaten, either using Din's Fire or by stabbing him (did anyone else notice this? The mirror of Link couldn't stab). FD Link, on the other hand, makes extremely short work of Majora, another embodiment of dark power.
I dislike both of 'em. I don't get why people say "But they are so bad-ass!!!". I think they are silly and overrated. I don't like that people think you should be able to play as dark link/oni link in the next Zelda games either. I just want regular Link!
Dark Link ftw! He seems to have a lot more depth that I can think of in comparison to Oni. Okay here's how I see it...
Oni is really powerful, being a god and what not. But really he's trapped in a mask. The only time he could really do anything is if Link was wearing the mask. The most that I ever see him doing is penetrating Link's mind and trying to take control of it. But in the end I think that Link would somehow win and Oni would never be worn again. (thus falling into the same dangerous mask category as Majora)
Dark, on the other hand, I see as being complex and troubled. He's Link's shadow (or Link's darkness brought to life if you'd prefer to see it as a balancing act), that means that he'll forever be plagued by Link! Dark would probably be so similar to Link in certain ways. He'll go on quests because it's Link's/his nature to help people (though he probably won't do much helping).
What if Dark doesn't even want to be evil? What if he wants to be a pastry chef at the palace? Or work at Lon Lon Ranch? But he's so blundering (the same way I think Link would tend to be, except in Link's case it all works out) that he ruins everything he tries to do?
I like them both. They both have mystery surrounding them and are not full developed.Each have powers that Link may or may not have. If they would expand on them at all though, I wouls want to find out more about Oni Link because I think his back story would have more depth in it.