Well, since everybody else is giving an open opinion I think I will throw in my two cents too...
With the 12x10 on my GT *every* single *super* deep, *severely* masked, or on edge coin that my friend's Etrac has found, the GT & 12x10 could see just as easy and just as well when checked before digging. This has been going on over several months of in field comparisons. Not one target the GT & 12x10 couldn't see just as good or hit just as hard, and all were seen just as good from the various angles and such around the target as we compared notes on them.
Conversely, I've also not found any super deep or badly masked coins that his machine could not also easily see. Also, one day I hit a deep whisper of a coin signal and was running full blast sensitivity. He was in Auto +3 and even though his sensitivity was running almost at max he could not hear the target until he put it in manual and raised it the last few clicks to full blast. That tells me both machines are pretty evenly matched in depth too. For that reason, I think all these Minelabs are pretty evenly matched with each other in terms of depth and separation *provided* the right coil is being used. You can't separate what the coil can't see separately, and in terms of depth on these Minelabs the only real way to push them deeper is via a bigger coil, so as far as any depth or separation of one Minelab over another my opinion is that it all comes down to the coil. After all, why put an 11" coil on the SE and Etrac when they could have stayed with the 10" coil? Reason I believe is better depth and separation than the old 10" Explorer coils.
So, I would tell you to base your decision on if you prefer computer controls and extra features that can be handy in certain (but rare) situations, or are you the kind of guy who prefers simple controls that give you enough control to still get the job done? There are some days I miss toying with and looking at a computer screen, so I'll add an Explorer II with a 12x10 to my line up some day again, but only for days I'm in the mood for a computer screen and such. The GT still stays no matter what for all the other days. There are guys who have tried all the other FBS machines but still swear the EII is the best on deep silver (at least in audio) for them. That intrigues me, so even though I've owned 3 EIIs, I'd like to add another to my line up again, but with a better coil then the old 10" Explorer coils which I never cared for.
I'd also add that it depends on the VDI resolution you want. If you want high coin resolution for certain coin types then go with the Etrac. If you don't really care what the coin might be as long as you can tell what zincs are when old coin hunting then the Sovereign is fine. I personally only want to know it's a coin when it's deep or shallow but in trash, as I've dug plenty of silvers over the years that read like a clad for one reason or another. If you want super high resolution from foil all the way up to copper pennies for things like splitting hairs on gold rings or nickles versus tabs and other junk, then go with the Sovereign. The Etrac's conductivity resolution is a scale of 1 to 50 with copper pennies starting at about 38. The Sovereign's VDI is roughly 60 (around where foil starts) up to about 178 where copper pennies start. Most Etrac guys don't pay attention to the ferrous number as it can range wildly, so conductivity is primarily what is used.
Another perk of the Sovereign is the ability to chest mount for water hunting. Mounted on the shaft for land hunting it's also better balanced than the Etrac, but still heavy like the Etrac.
A perk about the Etrac is it's got excellent Auto sensitivity. Set it at +3 and most guys never mess with manual as Auto will run pretty darn near the max you'd use at that site in manual anyway. The Auto sensitivity function on the Sovereign, while good, doesn't ride it nearly as high so it's not nearly as deep, so most guys use manual to hunt, which isn't a big issue to most experienced hunters anyway.
The Sovereign has better audio. More drawn out and detailed in target quality, but you can change more things about the Etrac's audio where nothing can be changed on the Sovereign but the volume of the targets or how loud the threshold is, or using Silent Search mode to hunt without a threshold. The Sovereign does have an excellent variety of target tones, though, like the FBS machines. Pretty much the FBS machines win in more control functions for the audio, but the Sovereign wins in terms of a long drawn out and detailed audio for telling target traits better.
That's my pros/cons based on my experiences. Others I'm sure feel otherwise...Both of course are excellent machines, and so are the Explorers as well. Can't go wrong with any of them.