If you look at the player rankings, Kael scores higher than Executioner in every location except Arena Defense.

Kael is a beast on Clan Boss, executioner not so much. Hes an all around good champion pretty much everywhere.

Kael, feed your executioner to him. :D

Go for kael over executioner. 

Kael can be used everywhere and to good effect, clanboss he's a very good early/mid game poisoner, he can comfortably farm campaign on brutal, even can be used in lvl 2O dungeons, advised to set him up in lifesteal/speed gear or lifesteal/crit if you're needing more crit to get to 1OO% total, he's also a very good pick for faction wars till later on.

Executioner is also ok, but falls off much quicker and isn't a very good choice for clanboss, he can farm campaign but it takes much longer, as well as dungeons but runs into the same problem, as well as not doing poison damage with later game does a lot more damage, he's also decent for faction wars. 

I agree with dragon, Kael was my first 6 star, he lead the way in the campaign, made his mark in arena(Got me thru) and also does decent damage to the cb!