Blue Kayn or Red?

I choose blue kayn because I just find it more fun to play, even before it got buffed.

I typically know which one I am going once game starts. You have to think about which will be available to you, and what is needed. If you have a melee heavy opponent team, then chances are you will be getting your red form filled first. If you face rang heavy, then chances are you will fill up blue first. Then you decide whether you want to wait 3+ minute for the other form that isn't offered.

Red:

Reasons for choosing - Your job is sustained damage and cc. You can even peel with your w if your adc gets in trouble. You work best vs heavy bruiser/tank teams.

Advantage - Tank shredding bruiser. This form is really good vs an hp heavy opposing team. He's good for sustained damage and with a lifesteal item like DD of BT he can really stay in a fight extensively. Disadvantage - Not as much mobility as shadow assassin (blue), so it's a little tougher to recover from a missed w. Harder to escape then blue form with your e if hit with cc.

Blue:

Reasons for choosing - The opposing team is range and burst heavy. Your job is to hit the back line and erase whoever the carry is and try to get out.

Advantages - Higher mobility and burst. You can keep positioning even as you activate your w. Your e allows for a quick escape if things go poorly for you. Disadvantages - Sustained damage isn't your thing. If you get focused by a group, you are pretty much done for unless you get out very quickly.