First talent As you can see in both specs the first talent is the same that means you need to farm and play safe as much as possible to collect your Globes and assist your team (Dying for trying to take a Globe is not good i have seen many people do it DONT DO IT)
Now for The Second Talent Nether wind is my most trusted choice when i play Kael'thas When Kael'thas first came out everyone including me was playing with Gathering power after multiple nerfs Kael'thas is now getting focused by the enemies you need to control them and have an easy way out in my opinion thats why blizzard buffed this spell of kaelthas and nerfed everything else its is also an amazing way to get enemies while they are trying to run and get the kill
Third talent Personaly i dont think you have a choice here this is a one way road
Heroic abilty Phoenix is my 1st choice here, in my opinion you can use it in many different situations. For example (Team fight--->Huge amount of dmg to all enemies. On Structures---> you can use it to bring down towers faster. To Dismount mounted enemies that are chacing you. On mercenary camps take them out faster with less life loss it has a very small cooldowon and can be used many times in many situations
Next Talent One of your best talents it will increase your dps and most enemies will target you.
Talent at lvl 16 Now depending on your enemies you must pick the talent if your enemies cant reach you easily you must think about going with the living bomb slow. now if the enemy team has Zeratul/Nova/Ilidan... or any other character that can jump to you and train you, you need the shield it is verry efective and will not give your healer a hard time to keep you alive
As for the last talent i always go with the Blink With or without the shield you need to be able to stay alive specially when the rez timer is taking so long

I hope you Enjoyed the guide Have fun Killing people on the internet!!!!!!!!!