I don't put much value in air tests other than giving you an idea of how a detector might respond to different targets. I certainly would not put much weight behind air teats depths. Too many variables even when testing 2 detectors side by side.
First off the ACS250 is a hot detector no doubt. Thats why it has the reputation it does, but you've got to remember having far less adjustments it is set for best performance from the factory. If the GTI2500 is anything like the Whites DFX its factory setting are quite mild aiming for better stability than performance.
Since I'm not familiar with the GTI2500 I'll once again use the DFX as an example.
Air testing the DFX can be very misleading to say the least. To make the DFX more user friendly for beginners its factory settings are very conservative, but make no mistake its a monster waiting to be let loose. If I remember right my buddy's ACE250 air tested deeper than my DFX but the results in the field were entirely different.
A little common sense can go a long ways. Think about it. You never see one detector claim to have more power than another. Why? because the FCC regulates it. So we can assume pretty much all detectors have the same amount of power, they just use it in different ways. The ACE250's weakness is saltwater. I can put Todd Rice's program in my DFX that makes it detect a gold ring at over 12" deep, but makes it useless in saltwater. So it might be safe to say the ACE250 has the same type of settings giving it very good depth, but for this added depth it gives up stability in certain conditions. In this case saltwater.
High-end detectors typically have more adjustments for a reason. It gives them the ability to still get good depth in more conditions than the preset ones. Really the major difference between most detectors, other than the obvious amount of control, is quality, durability, construction and weight.
For this reason there is no answer to "Which detector is best?" but which detector is best for me.
