The QA is "better" if you like a lighter, crisper 6.5lb trigger pull (with no second strike capability) and yes, it does have a decocker button, which is smaller than the regular P99 because Walther designed it to only be used if you are breaking down the gun and you needed to decock the striker. A much more elegant solution that the Steyr M/S series and Glock, of which you point the gun in a safe direction unloaded and pull the trigger to decock
The P990 was discontinued (at least in the US Market) in favor of the QA.
Regards,
James