Your question is a little hard to answer as they are not really 1:1 competitors. I have Superior, Ezdrummer2, Drumcore3, Addictive Drums2 and Native Instruments Studio drummer. These are in principle the same kind of vst instrument. GA4 however is a little bit different.
If you are looking for a “copy” of superior with a “better” sound, look elsewhere. If you are looking for a vst instrument that has a similar interface to superior, look elsewhere.
If you have missed some functions in superior that would make you use it in live sessions, have a look at GA4. If you want to manipulate audio files (no matter what kind) and build rhythms out of it, look to GA4. If you want to get a full drum track with intro, verse, chorus, bridge and endings up in a speed, look to GA4 (however EzDrummer 2 does this even better with the new interface).
If you rather have Steinberg do the composition work for you and don’t really want to spend much time laying down a decent drum track, go to GA4.
If you do not like reading what loops contain in respect of bpm, powerhand, and style and rather like using cryptic names like; “Lets go to the pub”, “I wet my pants” or “It is xmas time”, look to GA4.
BTW it is not true that 3. party grooves can’t be used in GA4. I have a huge library from Groove Monkee, Odd Grooves and Platinum Samples. They all work in GA4. It is not the easiest job to make them work, but fully possible, both to be used on the pads and to have GA4 play bak in Cubase. You can also build patterns from 3. party grooves and save them in GA4. I have posted a 15 page manual on this somewhere else in this forum. Not in the manual but also possible: To do the same thing with audio grooves. I have a lagre library of such from Beta Monkey and they play from the pads in GA4. However if you do not want to use this for a live session (or tweak it), there is no need of placing a groove an a pad. You would just import it into cubase directly.
With superior I think you already have the best vst instrument for drums available. Hopefully there will be an upgrade to implement the new functions of EzDrummer2. If I were to advice you, I would rather go for EzDrummer 2 than GA4. (If you do not want to chance that an upgrade of Superior is close). Or, take the money intended for GA4 and buy yourself some more grooves or a happy meal.
Drumcore4 is also “around the corner”(has been for almost a year). They are starting the beta testing as we speak. Drumcore3 was rather good and I expect a lot from 4. But it will “only” be a supplement to superior, unless they open up for 3. party grooves.
Another interesting drum software is Ministry of Rock from EastWest. The interface is more cumbersome than superior, but the sounds are great!