The XS2a has a switch for single tone or multi tone which to me most never used as the Sovereign are famous for their tones for IDing. The GT has a switch for threshold or silent search so you can select which you want to hunt with, with the XS2a you have to take it apart and switch a jumper wire to get silent search. The GT has also a switch for freq 1 or 2 so when you got into ares where there were some interference or where another Sovereign was you can switch freq to get a stable detector. Now beside the auto track and lock the ground balance to be used in all metal you can also switch the iron mask on or off in disc, when on it can see coins close to iron, but may false a little on iron, the XS2a it was on all the time.
Over all I feel the new GT is a bit more sensitive and you may see it false on iron Little more,but for those that know the Sovereign will also see a better ID and more depth out of the GT, those that don't know it well may not see the difference.
This is my opinion of the difference I see and have used the XS,XS2,the XS2a, the Elite and now the GT and even had 2 of the original Sovereigns and like my GT the best, so being you have a GT coming I feel you have the best of the Sovereigns.
One more thing the GT come with the new straight shaft to make it easier to swing.
Rick