I have both. Since you work in post I think that Absolute 3 would be far more interesting, depending on the job. Halion 6 is a lot more intuitive than Kontakt and is great for sound mangling, and both Halion 6 and the full version of Groove Agent 4 could be used to trigger edited versions of samples, all with their own insert effects.
As for their sounds, Komplete has a nice selection of synths but I like Absolute’s acoustic instruments better.
The sounds in Halion and Halion Sonic are almost all different from Halion Sonic SE, and are mostly higher quality versions of sounds from the Yamaha Motif XS. While they’re not super realistic, they have a nice “plug and play” quality to them. Halion Sonic 2 and 3 also introduced more instruments besides those, which can be identified by their macro interface.
The content from Groove Agent 4 is also all different from Groove Agent SE 4, and overall I like the stock synth drum kits a lot better than the ones in Battery 4. The paid expansions definitely sound better than the stock Acoustic Agent kits though. The Vintage Kit sounds the best out of the stock ones.