There's been a lot of discussion about Serana since Dawnguard came out. But in my opinion, Katria is hands-down the better companion. Let's talk about that.
1. No clumsy, moody backstory. Well, ok, yes, she's a ghost. But she doesn't moan about her family life or even complain all that much about her past. She allows you to focus on the task at hand instead of trying to draw you into a "my family's so complicated" trope.
2. She's an archer. That means she's not getting in your face, slinging spells around, or raising rabbits from the dead. Given how Skyrim's combat AI works, an archer is probably the least distracting/annoying companion type there is.
3. She has clear goals. Or, rather, goal. I'm not even sure what Serana wants. Also, I don't care. But Katria is a scholar on a mission. For the time you're with her, that drive helps propel you forward.
4. No attitude. I never get people who play RPGs and want to introduce a bunch of conflicting personalities into the party, for the sake of making the roleplay "interesting". Katria doesn't dump on the PC to prove that she's tough or witty or whatever. She seems mature & secure enough to not play games.
5. Short, succinct story arc. Man, I couldn't *wait* to get rid of Serana. But I was actually sad to see Katria go.
Lots of folks wish we had more companions like Serana. I just wish we had a few more like Katria.