The Original had a 8 inch coinsearch coil, plug in battery pack, could turn on or off iron mask. To me and my test it didn't seem as good as the XS, so I tried 2 and didn't like them as much as the later models.
The XS also came out with the plug in battery pack and the 8 inch coinsearch coil which many of the long time user like this coil, but was not waterproof, but didn't have the iron mask switch as everyone run iron mask on that I talk too.
Next was the XS2 which now Minelab went to the BBS coils so they were waterproof and came with the drop in battery pack and felt it would do as good as the XS did.
Now came the XS2a which was about the same as the XS2, but they added a toggle switch to single tones which 99% of people never used anyway as the Multi tones is what makes the Sovereigns do so well. I did feel the XS2a didn't have the power of the XS2 for me.
Next came the Elite which had the new battery system they used to this day on the GT, box was redesigned with a different mounting hardware and not the V clips used on the older Sovereigns. The control were changed so they were in a different position so it made it harder to use for those that had the XS series of Sovereigns. I myself didn't care too much for it as the targets sounded deeper than they actually were and the 3 I had used didn't get me as deep of coins as the XS or XS2 did.
Last one is the GT which is the model we have now, these came with a straight shaft here in the USA and the S shaft for other countries i am told. These came back with the iron mask switch so you can turn it off so the threshold is smoother for those I feel to help learn the GT, but also will not get coins in with iron trash as much. These also have a freq switch so if hunting with a Buddie with a Sovereign too one can switch to a different freq so they don't cross talk and also if in some electrical interference you could try a different freq to help out I find running in freq 2 works better and changing to freq 1 for interference. Now the GT also came with a silent search so those that want silent search could do this with a flip of the toggle switch. the older Sovereigns you had to open up the control box and move a jumper wire. In silent search it runs quieter, but also may not get the real deep targets you can get with a threshold. I never liked silent search, but see where it can be advantage to some area where targets are far and few, but then I switch back to Threshold when I start getting good targets.
The GT to me has more power and goes deeper than the Elite, Original and the XS2a and is more sensitive too which will cause a little more falsing on iron, but I can tell the difference with just coming from a different angle to see the target will move if iron.
The main thing is some don't see a lot of difference in the Sovereign, but those that get to know the Sovereigns well and learn what they are saying can and will see the difference in the GT over any of the other model Sovereigns. Where you will see the difference is in actual hunt on actual targets, not air test or reburied targets.
Those new to the Sovereign have to remember to be patience, learn the tones and get out and do actual hunting and do some digging and soon you will start to understand the Sovereigns and why we like them so much. Also remember you learn more each and every time out with these detectors, not so much with air test.
Rick