I'm not going to bother mentioning the Hatred (Y) beyond now because it doesn't seem necessary. It's not what you should be focussing on when deciding which upgrade to take.
Daemon of Khorne
Furious Charge and that's it. Really? In all honesty I feel a bit more effort could have been put into this one, and as is it's pretty useless. S6 is the magic number. At S6 you wound T4 (the majority of opponents you face) on a 2+ and you can cause Instant Death on T3 (the second most common Toughness value - taking popularity of Marine armies into account) opponents.
But what do you gain from becoming S7 instead of 6? Very little, really. You can penetrate the armour of vehicles slightly easier, which is kinda pointless seeing as you can just use your Smash attacks anyway for S10 and rerolls for penetration, and you can wound T5 opponents on a 2+ for a single round of combat, but how common are they?
Not common enough to take this upgrade over any of the others, especially with it being the cost it is.
Daemon of Tzeentch
Combined with Power Armour and Wings this is a very nice option. Slap on a Burning Brand of Skalathrax and you never have to stop Flying either.
Definitely an awesome upgrade when you consider that most anti-air is AP4, so you've got an extremely small chance of actually taking a wound.
Nice if you want to just keep on Flying around with the Burning Brand, or even if you decide to beat face with the Black Mace.
Daemon of Nurgle
Considering that Wings are essentially a must-have on Daemon Princes (you do want to actually get close to the enemy, right?) the only real downside to this option is that you'd have to make use of terrain constantly to really benefit from it at all. If you can manage that, however, it is slightly better than the Mark of Tzeentch when outside of combat. When in combat though, the Mark of Tzeentch is better.
I'd be giving this Prince some Wings, Burning Brand and nothing else. You don't want to rely on your armour save because there's really no point if you can use the terrain and you don't want to be in combat either because you have absolutely no benefits there.
A decent upgrade, but considering its fairly restrictive nature in actually benefiting from it, I don't know if I'd bother over Daemon of Tzeentch.
Daemon of Slaanesh
Rending is absolutely useless because you benefit from it in neither combat against infantry or vehicles. An extra 3" on your run is pointless because you're always Flying. Pointless. Even the Daemon of Khorne upgrade is more useful.
The Daemon of Tzeentch is really the only one that's very good, while the Daemon of Nurgle upgrade is pretty good but not great. Daemon of Khorne is something I'd only take if I were running a fluff-based Khorne army, but even then I'd probably just take Kharn. The Daemon of Slaanesh upgrade shouldn't be touched by a 10" pole, unfortunately. Slaanesh is my favourite god too :sadface:
A brief summary of the order:
- Daemon of Tzeentch (OP)
- Daemon of Nurgle
- Daemon of Khorne
- Daemon of Slaanesh