Good point about being able to change coils or headphones more easily on a Sovereign. You also don't need to waterproof the Sovereign so long as you mount the box high on your chest. That way you can hunt down to about your belly button and still be safe provided there isn't a lot of waves. I put mine in a plastic bag and tie it closed with a rubber band, then stick that in my Minelab hip/chest bag. That will at least save it from the random wave or splash, but I'm hunting the great lakes were we have less waves than the ocean.
The choice comes down to what you plan to hunt more. If you can see yourself hunting as much dry sand and parks as you do the water then the Sovereign would be the pick. If you plan to hunt more water versus dry sand or parks and such then the Excaliber would be the right choice. Then again, if you can afford an Excalibur then for another $300 or so you can buy an older Sovereign model and use that for the dry sand or land hunts.
Since I mostly land hunt the Sovereign allows me to do the occasional water hunt. I've got a homemade shaft for my land rig that is very light and uses the 15x12 coil and meter, so I set up the stock shaft for water hunts with the 10" coil always on that. That way I just unclip my control box from my land unit and throw it on the water setup or vise versa. It kind'a gives you the feeling of owning two machines with two setups sitting around to use.
Sooner or later I may buy an older Sovereign control box to stick in a waterproof box, or I could use one for my GT control box when I water hunt. Haven't decided on that yet and don't know if I'll need to go that route since I'm told most rings are found in waist deep water or less where my chest mount is fine. I'm no water expert so maybe my friend keeps telling me that so I don't want to hunt deeper, though.
I'm thinking that with a water proof box there would be two methods to try. One would be to mount the box high on the back of the shaft via an extension so it clears the water and creates less drag. The other would be to mount it on my shoulder via some kind of harness. I'm mainly looking at wanting to do two things- keep it out of the water for less drag and also for added insurance that it won't leak. Just because it'll be in a water proof box is no reason to tempt fate. I guess high on the chest or slung around high on my back shoulders would also be an option. Mainly I want a waterproof box that is just big enough to hold the control box so it's less bulky.