Kassadin or LeBlanc?

So I'm pretty new, I only started playing a few days ago and I'm looking for some advice. I use my free RP to buy Karthus on sale and have played him to the point that I think I'm fairly proficient at playing him. But now I think that it's about time to start branching out my champion pool.

I like playing mid a lot but I haven't played an assassin yet, so I took a look and Kassadin and LeBlanc seem like they would be a good pick. However their playstyles look very similar and I just can't decide which I should get so I came here looking for help. Keep in mind that I am sort of new so maybe neither would be a good pick, in which case I'm open to other suggestions.

Thanks for your time!