Quite different. Sonic 3 comes with a lot of extra content that might be of interest to people who will primarily use it in a DAW, and having lots of instances of a plugin isn’t an issue (so the UI advantages may seem minimal). There are more things you can edit and change up in the UI.
Aside from the gigabytes of extra content…for live performing, or in hosts where you don’t really want to mess with a lot of individual instances of the same plugin to get complicated layered or split effects…the UI offers some power user features that SE doesn’t have.
It has some interesting features for using it as a live performing instrument, either stand-alone, or in the host of your choice. I.E. Assigning entire multi-programs to a MIDI program change event. It has more tools for layering programs and/or splitting the keyboard (up to four per channel I think), and in ‘some cases’ it’ll even let you change up some sounds (load and manipulate raw layers instead of full Sonic ‘programs’) that don’t have a special macro (in this case, you’re using HALion layers that are basically mapped out samples or synth-tone genorators to build your own new program, and it looks and feels a bit more like the full blown HALion product).
The best way to know if it’s worth the money to you is to demo it. Info on how can be found here: Try HALion Now for 30 Days | Steinberg
Yes, the demo link is for HALion 6, but that bucket includes Sonic 3, so you get to try both.