The GT is the better Sovereign in my opinion. For someone new to the Sovereigns any of the Sovereigns would be good, but I would recommend the XS or the XS2 as I found these to be the better of the Sovereigns. The XS2a is OK too, but has that single tone switch you probably never use plus I never seem to get the depth I did with the others.
Now the GT don't come with a meter which most like to have as you can see with numbers what you are hearing for tones, so the tones that are close you can see the difference with the meter. This sure helps the learning curve for most I feel. I have seen a few good deals on used GT come up on the classified, so if you want one of them you may want to put a WTB on the classified.
Now on the XS2a there also have been some great deals on the classified and I would recommend getting one with the meter also plus see if you can find a 8 inch coinsearch coil, not waterproof but I feel it is the best coil to learn the Sovereign. I also like the rechargeable battery too and I know the GT come with them, so I would ask if you are considering the XS2a you have mentioned.
I notice the used Sovereigns are getting popular as you don't see as many as you used too on the classified and E Bay, I think some are realizing how good these detectors in if you spend the time to learn them.
I also like the S-1 probes as it makes finding the target easier so the best package deal would be the Sovereign with the digital meter( I like the 180, some like the 550) with a good rechargeable battery pack and possibly the second one or a Alkaline pack and the S-1 probe for under $500. I just sold a nice XS2 on a straight shaft with a new rechargeable battery, the alkaline pack, the Sun Ray DTI 2 with a brand new S-1 probe for $500. I am sure there is other deals such as this out there and these will be great detectors, but before you buy ask if the control box has any mods to it, some of these have been done and are bad.
The GT is more versatile and seems to be more sensitive so it seems to ID the deep ones better, but until you get to know the Sovereigns well you may not see the difference, so if you can buy a reasonable XS, XS2 or even the XS2a with the meter and the rechargeable battery it may be the way to start, that way if you don't like the Sovereigns you can sell it and get most if not all your money back.