I have the collection. I have almost everything Heavyocity.
Aeon is like the Evolve packs but on steroids.
great sounds very useful and mixing and matching from the rhythmic and melodic packs expands for a lot more possibilities. The twist, punish effect is like that of Damage. Aeon is like Evolve, Damage and mutations thrown in together with all new scripting and samples. there is no reused content from other packs - it's all new stuff. every package by heavyoicy can be used together and work seamless. I like it and it's one of my favorite Kontakt libraries.
Heavyocity generally only does a BF special or a spring sale. sales are rare or if they offer a new pack then those packs are on intro sale.
I bought the Aeon Collection in a 30% off sale. so $250 or something like that. totally worth it. for $399 it's a bit steep BUT it really is a great pack.
for $399 if you don't have Omnisphere synth and were leaning one or the other I would say Omnisphere. Omnisphere is probably my go to synth of all the ones I have, it just sounds amazing and it's really clean and easy to use.
I guess it boils down to how much money do you have, what type of music do you do, what do you need to add to your toolbox.