If that was a question between yasuo and zed or rengar and khazix, the answer would be much simpler. What i mean is that you are comparing a mid laner that can potentially play top, with a jingler who can potentially play top. They are different roles. So if you like mid lane, i say get yasuo, if you like jungle i say play rengar.
Now here is my final answer. I say rengar because he is less frequently banned and picked than yasuo is. Also mid lane is not easy to get as everyone wants to play mid. It's alot more likely to get jungle.
Last but not least i believe noone of the two is a good purchase. I mean your choice but i think they both have weaknesses. First of all they are all about dmg and dmg is expensive, so if you mess uo your farming or ganks you will be gold deprived and thus useless to your team. I like champions that can go either the dmg way or the tanky/bruisy way. Consider champs like vi, irrelia, elise, riven, wukong, vlad, xin-zhao, shyvana aaaaand JARVAN( i put that in caps cause jarvan is just fantastic, he has amazing dmg and good survivability) and more. Last thing, is that your timing is bad, yasuo is a nerf magnet, his mobility is too good so riot can't buff him without complaints so they prolly wont. Rengar ahd his trophy passive removed and his ult is too op for rengar to be getting any buffs.
