Understanding NI package covers much more ground.  We're just talking about "making modern music..."
 
So...if you've got some good drum kits, sample libraries, plugins, etc..
 
Which? (or who has both and has dived to the deepest depths in them...)

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

Well... I kind of look at this stuff and drool and probably should just get all of it.
 
I'm primarily a guitar player and have what I need in that department, so all the Guitar Rig stuff, etc...  I wouldn't need more choices of virtual Fender and Marshall amps, stomp boxes...
I also have nice Ric bass, some other nice guitars and a good assortment of hardware synths, JV2080 loaded with expansions.....
Of course I have all the Cake synths, and they seem pretty nice...z3Ta, rapture, Dimpro....  and Superior with a good handfull of SDX and ezd with the electronic expansion for weirdness. I had FM7, so I suppose FM8 is similar.
 
I just recently watched a bunch of Omnisphere vids (again and again) and all the Absynth, Massive stuff...
and I can't help but thinking my music with them would just be showcasing those instruments and not really anything creative out of me...   That's my fault, I know with this stuff. 
 
But then, this stuff is amazing and I love learning about it and it sounds great....and I'm glad I don't spread asphalt all day long for a living...(nothing wrong with that...I just prefer this musical junk instead).
 

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

Only difference between the men and the boys...

it depends on what you have/need /....
 
Omnisphere for me , is more solid than NI sampled based stuff (heavy to load , kontakt not that stable) ..but komplete you get all synths and some usuable fx !
 
In a lot of departement omnisphere is superior exept if you need orchestarl simple intsruments !
 
Camel Alchemy is also a beast

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.

For me, it would be Omnisphere in a heartbeat. But you included the phrase "modern music". Although that term is open to interpretation I'd take it to mean that the NI package would provide a much greater bang for the buck - for you. 

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.

FWIW...
 
I have K9 Ultimate and Omnisphere. If you are looking for meat and potato sounds (drums, bass, keys, etc.) along with a bit of synth sounding stuff, go K9.
 
If, however, you are looking for the ethereal and atmospheric definitely go with Ominsphere - it has that for days.  Also if you are in to sound design this would be the way to go as well.  It plays very well with Trillian and RMX as it is their "headmaster".  The arps are great to toy with as well.

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.