Blowfly.
If my memory serves me correctly. you have the X-Terra already. Is that correct?
I don't know too much about the Deleon, but I do know that has a fixed ground balance, which will hurt it on the wet sand. The X-Terra 50 should handle the wet sand better, than the other 2 you mentioned, due to you being able to ground balance it. I've used the X-Terra 30 a fair bit as well as the Ace. What I can tell you about them, is that the Ace will have about an inch of extra depth in both types of sand, it has a more reliably accurate pinpoint and if you hunt around play equipment a lot, then it would be the best choice, due to the fact that you can get in closer to metal better than most machines. If armed with the Sniper coil, then if experiment, then you will find that you can get as close as just over half an inch away from upright poles and if you hunt with the pinpoint button held on, you can hunt directly under metal. Even if the top of the coil is touching it! You have no discrimination when you do this though.
For regular hunting; if you have already got the X-Terra, then I think that it is as good a anything. I find the X-Terra's to very pleasant machines to use. It comes down to where you are at, at the moment. If you already have the X-Terra, then stay with it. If you have none of the above, then I personally would reach for the Ace just on shear performance. I do prefer the X-Terra's audio visual over the Ace though. We buy on emotion probably more than rational. So you'll have to figure out what you like in a detector. If you can't figure it out and have the spare cash, then buy all 3.

After all, a guy can't have too many toys.
Mick Evans.
P.S. The Ace will go 4 inches deep on wet sand with the standard coil.