I have enough ip to get one of them.

Zilean seems really cool with his "rewind" skill which reduces his cooldowns and timewarp ult.

At the same time Evelynn is just straight up awesome with her skills.

So who should I buy and WHY? Please say why

Oh, and keep in mind I could go invisible with Evelynn and then tele to another lane and surprise em I think thats awesome...Hmm so hard to decide...

dont even need to waste teleport for that, just get boots of mobility at lvl 1.

In early levels you would probably get more out of evelynn, she's a pretty good low-lvl/pubstomper.
Zilean is more teambased, and thanks to his passive he is banned pretty much 100% in ranked...

Don't get Evelynn.

Zilean. He's such royal piss off, that dude makes me pull my hair out even at loading screen. In the lane bombing you like a boss till you die/rec, gank? Well let's overclock my ass and away we go. When the bombs start to lose punch (depends how you build, AP or support), you are clocking your ult, imo one of the very best ults.
Zilean + Tryndamere in team, you might as well not bother past 30min mark.

On the other hand, Evelynn, you might get few kills early on, but then ppl get wards/oracles and you are done. Even if they don't you won't do anything due to your squishiness.