your choice... Komplete 9 ....OR..... Omnisphere + $500 (for ??)

I find the Synths alone contained in K9 to be outstanding.
But I'm a guitarist, so it's not my forte...

But I got my crossgrade for $199...and it was well worth the money.
Even at the full $499, if you add what you'd pay for everything contained
in K9 you'be at $2000 or more...and you also get the upgrade to K9U
when you're ready to take the next step...should you be so inclined.

But I've also heard that Omnisphere is a heck of a synth.

K9 has Kontakt, Reaktor, Absynth, Monark, Massive, and FM8 just in the synth dept.
Now add the effects, Guitar Rig Pro, Session Strings, MM-Bass, and a host of other
things...and you get the idea.  33 Products total.

So, it's ultimately what you're going to get the most use out of and what YOU do that would
be the determining factor.  I, myself, would do K9, which I already have done...only took me 
2 years to get there...LOL

I agree...it's like being a kid in a candy store...

Only difference between the men and the boys...

I would always go Komplete first depending on what you have. I'd get Kontakt at the very least if there are budget constraints. There's 50% sales every 6 months and 50% upgrade sales too.

Omnisphere is extraordinary though. You could make a living off the patches alone and when you own it you'll noticed that people in film & TV land are doing just that. Buy Heavyocity's Evolve and you can retire after a few keyboard presses.

It won't dry up your creativity but the synths mentioned are standalone beasts so you would have to go on a small learning curve to use them without them taking over. Press a key on Omnisphere and you feel like you've written a score. Press two and you've got a triple album.

Omnisphere is awesome - if you like what you seen in the demo vids you won't be disappointed,  but it's a different beast to something like komplete, so hard for anyone else to say which one would be best for your needs. Omnisphere is more about atmospheres, textures and synth weirdness rather than mimicking "real" instruments - so it's a question of what you think you need most.

mixmkr
 
Which? (or who has both and has dived to the deepest depths in them...)

If by "dived" you mean read the documentation and watched Groove3 video tutorials, then I have, but I haven't attempted much programming in either.  Omnisphere is much more straightforward to program than Kontakt, IMO, but you're stuck with the samples included (at least in this version of Omnisphere ... an upgrade of some sort is expected in 2014).  Of course, Kontakt isn't the only thing going in Komplete.  Programming ease depends upon which instrument we're talking about.  The diversity in Komplete is a two-edged sword.  Yes there's amazing variety, but then if you want to learn it deeply, there are many instruments to master that have little commonality in programming technique.
 
If you like the availability of 3rd party sounds, Komplete wins by a mile.  There is a small amount of 3rd party sound content available for Omnisphere, but there's an ocean of it for Komplete.  Much of my favorites in this category would be the sample banks for Kontakt from the likes of Hollow Sun and Hideaway Studio, all at very reasonable prices too.

You can produce amazing atmospheric or soundscape type sounds with any synth. You do not need a special synth to do it at all. In fact it is better to construct your own ambient or atmospheric tracks yourself by building lots of original sounding interesting layers yourself and mixing them that way.
 
Pressing one note down in Ominsphere with a great atmospheric sound is not the way to go because you have in fact done nothing to create that sound other than listen to presets and choose one. Also every man and his dog also has that sound so beware. Editing that sound of course is a good thing to do but if you are using stock atmospheric sounds then you are really doing nothing and there is nothing original about doing it that way.
 
Brian Eno (master of ambient and atmospheric soundscapes) actually says in one of his documentaries that you only actually need a dozen or so synth sounds to be able to create anything and I tend to agree. Having too many synths and sounds is actually a hindrance not an advantage.
 
Komplete can easily create highly original ambient or soundscape type pieces. You have all the tools to do that, it is just how you go about it that is important. Plus it is loaded with bread and butter sounds too. So in some ways you have got the best of both worlds right at your fingertips.
 
If however you are loaded with bread and butter sounds already and want to get into atmospheric type tracks then maybe Omnisphere is the better way to go. I am sure it is a beautiful instrument. The thing for me with Omnisphere would be really learning it and creating my own patches. Using the existing patches to backwards engineer how those patches were made in the first place as well would be the way to go too.
 
Your creativity and cunning easily out perform any instrument that you may own. Heck I even heard a great ambient piece that Craig Anderton did with just guitar and weird spaced out effects and it sounded like something Omnisphere would produce so it just goes to show it is the ideas and process that are far more important. I have produced incredible ambient tracks just using sounds found around the house and done some tricky digital manipulation to them.
 
If you get Komplete I will give you a copy of my DX7 library which is over 7500 patches now in size and loads directly into FM7/8 of course. There are some pretty weird spaced out sounds (as well as great bread and butter sounds) in that library as well. I have got every DX7 sound that was every made I think and once you start auditioning them you will realise what an incredible instrument that was even for its time. Another good thing about Komplete is the fact that some of those instruments actually sound quite different from each other and I am referring to their sonic signatures. With Omni you will be getting one sound more so compared to a whole range of sound signatures with Komplete.
 
The synth itself will not turn you into a great composer. Only you can do that. If your music is great then it will still sound the same using any synth. Some synths however are very inspiring to listen to as well and that also needs to be taken into account. Camel Audio Alchemy is one of those for me. That is a very special instrument too and does things and sounds like no other as well. Especially the groove based material. Talk about amazing!
 
 

One thing I really like about Absynth is its patch mutation feature.  It's a quick way of generating new sounds so your friends don't say "Oh, I see you used the 'Sunrise on Io' patch on that track."
 
Also, check out the Hollow Sun and Hideaway Studio pages.  They create some unbelievable sounds, sometimes out of vintage equipment, sometimes "Oh, I sampled my doorbell running through the ensemble section of an ARP Omni and through the amplifier salvaged from a 60s era console hi-fi onto some equally vintage tape."  If you like those kind of sounds, it's a strong point in favor of picking up K9 for Kontakt.