I bought the older bundle, so I have all the expansions except the last 3 or 4.
It's a tremendous synth, and to a large degree it's smaller library install footprint is due to it's architecture differences from Omnisphere. It does more synthesis through CPU calculations than Omni generally. Some of the expansions are pretty small overall and only include new oscillator waveforms and settings data but still have a healthy amount of new patches.
The one thing that sometimes gets overlooked in the comparison to Omni is the fact that Alchemy allows loading of user sample data where Omni has nothing comparable. It could be as simple as loading your own oscillator waveforms or any sample as an oscillator source. But it can load entire multisampled patches too, they just have to be in the sfz format. I use Extreme Sample Convertor for that.
I've taken a lot of the old E-MU and Ensoniq multis I have and converted those. With the extensive sound design/shaping/modulation controls Alchemy has, you can take some basic sounds like the ones I mentioned to radically new places. It's good fun and when using solid multisampled source material you get some very usable results, and probaly would have to tell somebody "Yo that's actually the Mo Phatt ish tweaked into outer space" in the example I'm using.
I think Omni sounds a little better, but I'd say that extra definition it has is down to the 50 GB library and the larger multisampled patches having so much data. By no means does Alchemy sound bad, it sounds great.
I might say that Omni has a little bit more patches that are ready to drop in a mix as is, more bread and butter ones, Alchemy has more spacy and soundscape sounds. But I don't want to give the impression that either synth is only those respective styles. I think Alchemy just is set up to be a little more on the sound design tip, Omni a little more on go to sounds you can find quick style.
I love having both but it's hardware level bread you have to break to have both, and even one or the other is still pretty pricey. I'd probably use the free Alchemy Player and go through every single patch if I was considering an Alchemy purchase. They give you a decent assortment of ones from the different libraries to get a good overview of how useful or how much value you might get out of the whole bundle.
