It's a playstyle preference. Morgana is support, Kassadin is an assassin. Morgana's crowd control is a skill shot, but it's more powerful than Ryze's if it hits. Kassadin can squish people rather easily, and has a cone slow and a silence as well.
If you solo a lot, probably Kassadin. If you play with friends, probably Morgana (unless one already plays support, it's usually not a good idea to have too much support on a team, you'll get rolled by gank squads.)
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Morg is a great support character, while kass is more of a anti-mage/mid game ganker / soloist.
Morg will rely on good teamwork since she has a built in cleanse spell (frees up a summoner spell on a teammate if you work well with them, maybe even more teammates will not take cleanse since you can debuff them and shield them) you also have a snare which makes running from a gank impossible. And of course the stun is amazing on her ult (slow + stun) She won't be racking up kills, but she will get the assists a lot of the time. And if they actually are playing with a heim you can walk over to the lane thats having trouble, cast the little black pool of death and walk outta range and repeat til they die (might force heim to waste heal and rally in the process even) But thats just bonus. She can farm fairly well with her black pool even (she can be played quite well as a tank support, so you're even that much harder to kill so you're even better at supporting :D)
Kass on the other hand will destroy teams single handedly mid game. He can chase almost any hero (save singed/yi/eve/twitch/zilean... all the insanely fast ones or with great espacing abilities)... flash will not save them as his teleport will catch back up. and he can always slow + frozen mallet. Fun hero, but honestly in team battles he has yet to have a use. He is eve who can't cloak, but flash constantly to gank. He is a bit sturdier, but not quite as strong burst damage...
If you like to support more, pick morg. If you like to see your name pop up a lot pick kass. Honestly if you trust the team a lot, i'd pick morg and learn her ways since a majority of people play as carries/gankers you can be that person thats helping out the most by adding a bit of variety. :)