AT Pro is deeper. It's not a depth demon, but depth is decent. The Ace series is not as deep and doesn't unmask as well. Caveat: I only used the Ace 250 when I traded for the day with a buddy, so my experience with the Ace isn't extensive. But long enough to know I didn't care for it. I never liked the old Garrett binary audio and to add insult to injury, it's yellow. Bright fricking canary yellow!
I've had a Pro for over 7 years. Like I said earlier, it's not incredibly deep. But in my moderate soil (GB almost always mid 70's to mid 80's) it's more than a 6" wonder. With the 5X8 coil, my deepest measured small coins (1c or 10c) rounded to the nearest 1/2 inch are at 7.5". With the stock 8X11 coil add another 1-1.5" of depth.
There are some ways to get more depth with the Pro. First of all a technique that works with some other detectors too is the fast back and forth shake with the coil over the target. If a softer partially broken high tone gets solid or mostly solid, more often than not you're golden with copper or silver. Or strangely enough a deep nickel. For some reason deep nickels often give a predominantly high tone with the fast wiggle. Maybe the roundness?
Dropping the GB 10-20 numbers below balanced adds a bit of depth when conditions allow.
When coils are switched on the AT PRO, they say when first turning it on to do a long press of the power button til the beeps to sync the detector to the coil. Didn't realize you were supposed to do that the first few years I used it.
The audio is where it's at with the Pro. It's like being fluent in a second language. That's why I've still got it around despite:
Depth below top tier
Mediocre build quality
Too small screen and no backlight
More difficult proprietary connectors
Bad ergo with the stock coil - nose heavy