They are both twins!!
but Tritannus is evil and Nereus is not!!

Kinda like Nereus more.

Before he became a mutant mermaid that is. I like Nereus better, but the pink hair isn't hot to me.

I like Nereus more because of his personality and actions more-so than his looks. Of course Tritannus is evil and what-not, but he's so whiny and annoying. He doesn't seem to really know what he's doing. He's just derping around in the ocean and hoping to find a guardian somewhere so he can get to the Infinite Ocean. He never does anything himself (Save for turning creatures mutant—which isn't new—and taking the power of the Selkies which are almost immediately restored) and always calls upon the Trix to take the Winx down, give him more pollution, find Daphne...I mean, yeah, I hated Darkar for making them steal the pieces of the Codex, but at least he made them do something. With Tritannus they just sit there and do nothing until he tells them to do something...and it's usually something boring anyways. The Trix are powerful and apparently your sidekicks. Use them well, you twat. Don't just reduce them to petty work.

Nereus on the other hand, is brave. He steps in to fight a defend his family, even getting hurt in the process. Though I don't really care for Neptune favoring one child over the other (Because that's just great parenting right there) I can see why he would choose Nereus. Tritannus is a cry-baby-bitch. If he wanted people to see him as his brother, then he should do something about it. Other than plan to bring the "demise" of his family. Psychos always end up in prison one way or another, even mer-men should know this!

Neither are handsome but if I had to choose, I'd pick Tritannus.

They have the same face. I think they're both good-looking, though.