X2 , X2GS, GUS or GUS-on-Orpheus2 all will sound different as they use different synths with different sound banks.
All can load custom sounds/presets but the difficulty for each varies - I have no experience in this so no idea if something is more easy or better to use. AFAIK to create custom banks for the X2 line of WT boards one would need the DREAM SDK that is a hardware/software combo. No idea if GUS tools are freely available or not for this purpose.

Since you already have sound in your games and I assume it does the job for you maybe all you need is a gameport->midi adapter
If you want to experiment with different synths and how GUS sounds then it's another matter , you need to decide what interests you , youtube has many recordings from various soundcards/devices.

I’m really only using the existing audio for sound effects, not music. For music, I’m looking for maximum flexibility in being able to assign whatever wavetable samples and external synth/module patches I can at each instrument being played. I’ve a host of outboard gear, which will be fun to use, but the convenience of loading up samples internally on the onboard wavetable appeals so I’m not working with external sampling equipment.

Essentially it sounds like they can all do it, guess I just need to review the utilities now see which one will make life easiest.

Thanks for the info.