Kassadin or Karthus?

Neither.

Malzahar

Kassadin is much less a mage and more so an assassin. Karthus requires a special mindset to play. If you had to choose, go with Kassadin. He's actually not bad as people may claim, just he requires you to be a bit careful in the laning phase. However, you can roam very well and snowball.

Karthus is thenbetter of the two imo, he does far more damage for a start.

Both are in a bad spot simply because champs like ahri brand and zed are popular atm

What style of Mage do you want to play most is the question you should be asking yourself. Do you want a somewhat tanky control Mage or do you want an assassin Mage? If you want an assassin mage more, go Kassadin, but if your playstyle prefers control mages, go Karthus.

kassadin is actually fun so play him.

Dont listen to the haters saying he is "bad" they just dont know how to play him correctly he is actually pretty good

all the people saying he is bad are rengar spammers who only play broken champs to win whilst kassadin is balanced and you feel accomplished when you destroy people with him

If you play kassa din I automatically report foe unsportsmanlike conduct.

Hey, man! I'm in high Platinum, and I've seen both these champions played at a high level.

While neither one are broken, they're quite playable. They take a lot of practice, though. So, if you want to be good at these champions, you have to practice a lot before you see some results. They're both relatively safe laners, meaning they can be played into a wide variety of match ups and be okay. However, AD mids just dominate against them. As for who you'd rather play, they're both very different. Karthus is a hypercarry that stays in the back of team fights and just dishes out tons and tons of damage from afar. Kassadin is a durable assassin who can dive into the back line with ease and get out, if he manages to stay alive. I hope this short description helps you.

Kassadin = high mobility (at lvl 6), melee anti-mage you should pick him when you are sure you will be facing a magic damage dealer, very hard lanning phase from 1 to 5, where your objective is just to not die.

Karthus = long range poke, you use your Q spell like it is your auto attack (you gotta like spamming that button), you want people to stay way from you, if they try to get close use your W wall to slow them, if they happen to reach you, turn E to deal some aoe while they are hitting you, you gotta have lots of map awareness to know when to use your ultimate.

Neither save up for or . All much better champs IMO.