I did a quick comparison video of the two plugins.
No mixing/mastering wizardry, just raw guitars into ampsims, both using the same IRs. Some drums and bass on top, et voila:
Tell me what you think.
BTW, that’s my first published original track.
As a proud Sigma owner (so not biased at all hahaha) I’d say it did pretty well 
In fact I think I prefer the Sigma, it’s a bit more “classic rock” if you know what I mean
I too own the Sigma, and the first time I was comparing the two with external IRs they sounded almost exactly the same for me, with NDSP being more alive in top end as well as more “life-like” and better quality overall.
But I must say Sigma is still good and has something interesting going on.
But main difference is NDSP Omega is brighter and has more gain, plus you get their fancy cabsim.
For the value though Sigma is a clear winner here.