I own both. The Ace for a few months, and the 2500 for a few weeks.
My favorite features of the GTI:
TreasureTalk - I have it set to tell me the conductivity while scanning and size/depth when pinpointing.
Imaging - While not perfect, it does stop me from digging up most aluminum cans. I've been digging everything to test it's accuracy and so far it's pretty darn close. I do however dig quite a few can shards that are coin-sized
It's a bit off sometimes on shallow targets.
Pinpointing - Very accurate! A few waves of the coil, and it's dead center.
Caveats:
Weight - It's heavier than the Ace. Though not a problem for me, for some it may be a burden.
Backlight - Not that it's a problem, but I would prefer a simple on/off switch instead of navigating a menu to turn it on and off. I've left it on a couple of times after a hunt and didn't realize I had left it on the next time out during the day.
Sensitivity - The Ace is better at getting closer to metal playground structures, although I have not tried lower sens settings than stock.
Ace advantages:
Value - It's a great machine for the price, and at 1/5th the cost, I can leave it in my vehicle during the day for an impromptu hunt anytime. The GTI stays at home until I'm ready to use it. I'm a little uncomfortable leaving a $1000 machine in my vehicle vs. a $200 one.
Lightweight Light enough for anyone to use, easy to pick back up to repinpoint if needed.
Ace Caveats:
Soda Cans! Being from the land of the 10cent can deposit, there's still plenty of them discarded, crushed and in the earth. I know that you can ground balance/pinpoint and get a good idea the difference between a dime and can, but it's sometimes tricky. The GTI nabs it with a press of a button.