Who is better? Arvis or Katarina?

And Arvis also is a passive healer on his own

I like Katarina better,
just because you can't get a +10 Arvis and he can't have any boons/banes.
Else, I feel that Arvis would be simply better :
- Has a legendary weapon with res play and atk ploy in it.
- Recover Ring is like a Renewal 4!
- Has Def Ploy 3 in is base kit

Well, honestly they don't fit the same role ^^
Katarina's tome allows her to play around her team position to get more buffs, thus she can reach great spd and pretty good atk.
For damage, I'd say she can do better because of spd mostly.
Arvis doesn't require as much positioning from his ally but needs to position well for debuffing.
He is pretty tanky and his damage is quite high, but without heavy buffs from his team, he won't be doubling a lot.
Also, he can be a very good passive healer with ardent sacrifice and can run fury without being scared of recoil.

Overall I absolutely love my Arvis and I think he is an awesome unit, but Katarina is good too so it's up to you to decide who you need most.