Hi jensman.
I've had the 1500 for around 3.5 years now and I'm pretty happy with it. I've never laid eyes on the 1350 though, so can't you you much info in that regards.That said, I from inquires I've made and reading this forum, apart from the imaging verses profiling of the 1350, they are supposed to be pretty much the same machine. I will go over some of the minor differences that I'm aware of.
On the up side, the imagining is a little more refined on the GTI's over the GTP.(5 segments verses 3.) This can help separate coins from other similar sized object better. ie, screw caps from coins. you do need to have 5 to six inches on the depth meter when sizing for these sort of targets to get an accurate separation though. Another coil that the GTI has that is not available to the GTP, is the 5x10 DD scorcher coil. I most often prefer this coil over the imaging coil for a few reasons. Target separation is markedly better (faster recovery speed due to not using the imaging circuitry) Also, the others significantly better feature of the 5x10 coil, is the audio is more distinct. By that, I mean that on items smaller than screw caps ie coins, they give a proportionally shorter than full tone when you pass over a target, depending on size. Concentric coils are easier to pinpoint with than DD,s so that is worth taking into consideration. The other minor thing that is easier to see on the screen of the GTI is the ID icon. It is a full block where as the 1350's icon is more like a narrow arrow head in shape.
The only 2 downsides of the GTI are 1) having 2 receive coils can make the GTI's twitchy if the ground gets noisy (mineralised eta) It does have a chip in it which will stop it going into a full overload, but if the ground is too mineralised, then all targets will read as a 3.5. The other major draw back, is the poor recovery speed of the imaging coil. If the ground has a lot of targets, then there every chance that you are going to miss a lot more targets, than if you had the DD coil on.
At the end of the day, yes it's an upgrade, but not a big one.
Personally, I would prefer the concentric coil of the standard GTP as I've always been very fond of this coil design on Garrett detectors as the audio fade you get when you come off a target in pinpoint, works very well at telling me the size of the target that is under the coil. It is for that reason that I've often thought about owning the GTP. Now the AT Pro is out with that coil available and with a superior audio system, i would lean that way if I was to buy another detector. (Often tempted, but what I've got works for me now. I may well buy one down the track.)
HH.
Mick Evans.