Zed V.S Katarina (Who should I get?)

Zed is one of the most mechanically intensive champions in the game. Maybe that's what you're looking for, but in all honesty, sub-30 players will probably find Zed more frustrating and confusing than fun.

I'd say go with Kat. She's much, much easier to perform well with. You won't be gaining much experience in the way of mechanical skill, but you WILL learn about positioning, when to go all in, managing your cooldowns, and optimal roaming tactics.

Neither. Please don't buy either of those champions.

First of all, Katarina is basically banned out of every ELO. She's just a dumb champion with an easy and stupidly powerful kit. If you find yourself playing her a lot you'll probably get a lot of verbal abuse for it, and you'll also probably get bored pretty fast since her kit isn't very complex or fun to play.

Zed on the other hand, you will find yourself winning lane most of the time and going on to lose the game anyways. His teamfight is weak and split pushing is hard to coordinate in soloqueue since people tend to fight regardless of whether they're grouped or not because they 'saw an opportunity' and got greedy or something.

Get Talon. He's like a more reliable Zed that isn't as feast or famine as Katarina. That's my recommendation at least, the best thing you can do is play the champion you like more, you will improve more since you'll be playing and enjoy playing them more.

Why not both? You can effectively get kat at any time due to her being relatively cheap (ie NOT 6300 IP) and would take less time to get than it would if you bought kat first and then went after zed.

Zed getting a nerf

I like to play an assassin type champion who can carry a team, and a champion that will help me develop into a better, more skilled player by the time I reach level 30.