Nexus has a good sound engine and FX with it's internal DSP code and the fact like the other two synths uses large amounts of multisampled WAV data to generate it's sounds.
But it is extremely limited in what you can do to edit the sounds, and personally I find a lot of the sounds FX to be extremely over the the top and frankly cheesy and garish. I see it as a mainly a preset machine and a vast majority of the sounds are intended for trance and other electronic genres. Again to be frank I think it's wide use is is responsible for a lot of everything I despise about modern mainstream preset, by the numbers hip hop.
I have met countless people whose entire setup is a FL and nexus crack and a mouse with any keyboards or pad controllers collecting dust as they just take trance arpeggios out of Nexus and program one note to trigger, grab their awful Lex Luger downloaded kit and use thin poorly sampled 808's 100% of the time.
Again it sounds lush overall but you're too trapped in what the give you, and toning down the stock FX and a very limited amount of modulation options are to me a hindrance.
Also it takes well over $2000 to have all the expansions which to me is insane when you can buy a an Access Virus TI for that kind of money and have one of the best digital hardware synths ever that can do 16 tracks of sounds, work as a plugin all with zero CPU hit.
Omni and Alchemy and Alchemy to me are much better implementations of a similar design philosophy in that they use large amounts multisampled WAV data, but have every synthesis and modulation control you would every want in synth.
Both also shine in that most major styles of synthesis are represented. In both cases one is not limited to just presets as the Oscillator WAVS are all available to mix and mix and it is quite easy to study the factory patches and library patches to see many ways you could use their constituent parts to create your own patches, or just tweak and alter the factory/library ones.
Omni is more expensive but I think has more factory patches than Alchemy, certainly more ones you can use right away as is or with minimal tweaking. By that I mean more bread and butter sounds that drop right in a mix, or are more "musical" in their tone and usage. Alchemy has more "spacey stuff" that while amazing may not be the first thing you looking for for say a fat bass sound or a Moog sound and those things. Omni has countless patched like that sampled from the real machines.
I think Omni sounds slightly better with a bit more detail and separation in the frequencies and tones. Many of the patches have that "essence" or tone of hardware. Spectrasonics owner Eric Persing worked for Roland for years and created the patches on more than few classic Roland synths in time past. He is a sound design genius and a very helpful guy if you need support or catch him in the Omni section at the Northern Sounds forum. Omni's other patches are created by a great assortment of some of top people in the world doing synth sound design.
Alchemy also has some great people doing their sound design and while I might give Omni the slight edge Alchemy still sounds fantastic. If you want to invest in few of the libraries it can quickly put the price much closer to Omnisphere in order to get a similar amount of patches that Omni has at the $479 bass price.
The Alchemy bundles do save you a great deal of money if you want to acquire the libraries in addition to the main package.
One advantage ALchemy has to differentiate itself over Omni is that it can load user sample data as Oscillator sources as well as take entire patches in the .sfz format. So you can take any .sfz multisampled patch and load the whole thing into Alchemy and take it to completely new places using Alchemy's FX, sound morphing features and modulation routings.
For example I converted all my E-MU and Ensoniq patches that are mostly typical bases, drums, string and general synth sounds and load them in Alchemy and within minutes you can have something radically different with near infinite possibilities to tweak.
You can also DL the free Alchemy Player and use that to play any purchased Alchemy libraries, but the editing is quite limited.
Both Alchemy and Omni can be used with an iPad as well. Alchemy Mobile is a basic versions and an in app purchase unlocks all features. If you own the desktop version of an Alchemy library you can DL the mobile smaller counterpart for free.
The mobile app over wifi/Bonjour can be used to control and tweak the desktop version.
The Omni remote app is for changing patches and controlling FX and I think navigating the various tabs/pages of the plugin. It is free.
The stock install of Alchemy is just over 5GB, the Omni one is over 50GB. This is why Omni has more patches and a more defined sound overall, but again Alchemy does well with a smaller data set and sounds very good.
So due to lack of user customization and how much it costs to have all the libraries I put Omni and Alchemy far above Nexus even though Nexus does feature a quality sound engine.
I own Omni and Alchemy and have about 80% of the Alchemy libraries. I have used Nexus many times but do not own it for the reasons I mentioned.
That's all I got.

I hope it's enough to help your decision. I would rather try to go into detail than just post "Get [this one] it's dope!"
