If it is a XS or a XS2 they were the best of the older Sovereigns I feel, the original I didn't care for as much as these 2, but it works good too. The Explorer SE will probably have the best depth if you have learned it well just like the Sovereign as to me it is easier to use and can hear the signals better with the Sovereign and the depth is good, but like the Explorer you have to go slow and listen close to get the depth.
The Sovereign GT is the best in my opinion of all the Sovereign if you get to know it well like the E-Trac is much better than the Explorers too.