I own an Ace 250, and a GTI-2500. The Ace is very hard to beat when it comes to hunting coins. You do have to operate it in all-metal mode if you want to find nickels, especially clad ones which show up most of the time in my soil conditions as a pull tab. Mineralization isn't easy to deal with either, but the Excelerator coils do help.
The GTI-2500 is heavy, especially if you use the larger 12.5 inch coil. A bungie support is almost a necessity for long hunts. For US coins at least, the 2500 is very accurate in depicting what it's looking at. In my soil conditions, I can tell the difference between a clad and a silver quarter rather easily. The same thing with pennies. However, any nickel ends up the same place on the scale. Like the 250, if you want to find rings, you almost have to use all-metal mode, and just dig what you're not sure of.
I can't speak for every type of soil conditions, but here in my neck of the woods (Roswell, NM), the 250 will find a quarter down to about 10 inches, and the 2500 well past 12 inches. Thus the 2500 will give you a slight edge when looking for really old silver. At least it has for me.
Alan Applegate