Kassadin is a mid anti-mage assassin. He is good against AD but even better against AP. Malzahar is good against both AD and AP equally.
All depends on your playstyle.
Kass is the one who never gets killed if you use him right, but also needs excellent positioning in order to get kills. He is very weak before Lvl 6 and after he is a complete beast.
He has a instalock silence, passive for mana and AP into basic attacks, a cone slow and damage which needs six charges and a constant gap closer damage tool.
Malzahar is one of those champs which benefit the team AND is a great duelist. Malzahar is also anti tank as he has percentage damage. Malzahar needs excellent positioning too, but is easier to get into position. He has a silencing wall skillshot (hard to land, which is why its fun), an AoE percentage damage tool, an instalock DoT and a suppressing DoT.
Kass benefits the team through constant burst damage however he can be extremely mana hungry. Ulting into team fights will help with your burst, but also risk yourself due to the fact that you are melee. His Q is quite nice as if can stop running opponents using Flash or their own dashes, his W is nice as the damage scales on AP and works against turrets+inhibs, his E is slightly tedious for the 6 charges, but it is nice for farming/slowing enemy runaways. His ult is the most troll. Flashing everywhere with a 5 second cooldown (despite mana cost). Easily one of the funnest ult in the game, however if you are not careful, you will screw yourself over.
Malzahar can stay a distance away while landing a silence which can silence the whole team if used at the right time. He can burst wall damage and AoE DoT Percentage health which is great against tanks. His E is best at killing runaway champs on low health. Usually I se someone low, I give them my E and I go continue what I was doing, expecting a kill (which usually comes). His ult is quite useful in many ways. His suppression can be used to kill off running opponents or even better, suppressing the enemy carry right in the middle of your team.
Overall, Kass is the mid guy who shuts down people to get extra money (or normal kills)
Malzahar is the mid guy who helps to kill anyone or get a kill himself.
I have both and I love them both, however I still prefer Malzahar, but that's due to the fact that Malzahar has the "Can't Touch This" dance AND his Voidling dances with him :P