Omnisexuality is defined as an attraction to persons of any gender identity (incl non-binary), but gender does still matter for an omnisexual (so in my case for example, I'm attracted to everyone, but more strongly towards men and less towards non-men). Bisexuals are attracted to men and women, but could also be attracted to non-binary people (but not necessarily I think).
So I'd say there's very little difference between them, if any. It's more of a vibe-based thing, whatever label you like the most.
Ecosia mostly