Currently I'm trying to choose between 
or 
. I'm no stranger to mechanically intensive champs( Lvl 7 
) so from a learning perspective I don't have an issue investing the time to learn .
on the other hand feels fairly safe from nerfs and seems to have a lot of carry potential, if only because no one knows how to play around him anymore
Karthus is basically an AFK ganker if I recall. He can quite easily stay in lane and farm for day, while still maintaining map pressure. Of coarse you have to be able to farm.
I like him, But I usually use him in close quarters game modes, such as the TT Hexakill. Come late game, he'll work way better if you can get everyone to stay still and in your E range.
Who ever u find more fun? Regardless if Azir gets nerfed more or not, hes gana be strong until they decide to rework him. His kit is just that good. Think about it, hes been pretty much nothing but nerfs since release (that and bug fixes) and he's STILL strong AF.
Well do you wanna have fun or win. Have fun: Karthus. Win most of your games: Azir.
Both are high skill champs, but the meta favours azir heavily. Both are fun. GL
Karthus for afk farm and late game monster, Azir for zoning and flashy LCS plays
actually anivia might be good to learn, she has great waveclear, AMAZING burst and she is a control mage
but if ur set on one of those 2, Azir for zoning and poke, Karthus for late game
