Some reflections ... Never used a GTI1500, but I have both a GTP1350 and a GTI2500 ... (was already using the GTP and then found a good deal on the GTI, and couldn't pass it up.)
Comparing the two, the GTI2500 is just a little easier to use, because the imaging (size and depth) comes automatically during the pinpointing swings.
With the GTP1350, first comes the pinpointing to center the target, and then releasing the pinpoint button and continuing to swing (7 inches each side of center) gives the target profiling size and depth.
The GTI2500 does it all in one operation, saving a little bit of time and keeping you right on the target center.
But I wouldn't complain. They are both sensitive, accurate and stable detectors.
They do it with a 3-element coil (one Transmit and two Receive elements), compared with 2 elements on other detector coils. That makes the coil a bit heavier, but gives you the imaging.
As a note, also I have a Teknetics Gamma. It's sensitive too and the target numeric ID is helpful. If you swing over the target in different directions and the ID stays fairly steady, you have a good target. It's a little better than the "segments" on the GTI/GTP display. Also it's much lighter than the GTI/GTP, and that makes a big difference for the arms and shoulders after some hours in the field. It has some issues with EMI/RFI, but otherwise it's good.
Also the GTI/GTP have a salt elimination mode, not found on the Gamma. (Seashore beaches here in Florida.)
My ideal detector would be a combination of the two: numeric ID while searching, target imaging while pinpointing, salt elimination mode, and light weight.
Hoping it's helpful.