I have owned quite a few Garrett detectors over the years. From BFO's to the GTI's, like the 1500 you mentioned. In fact, I was a Garrett dealer back in the 70's. One of my favorite "early" metered detectors was the Garrett VLF/TR Groundhog. I dug lots and LOTs of coins with that one. So in that regard, I won't say anything bad about Garretts and I won't try to talk you out of trading. But you asked for reasons, so I'll offer my reasons why I would recommend keeping the X-Terra.
The Garrett 1500 has been around since 1998. Some would call that "proven". Others might consider it "old technology". The X-Terra has been around for about 3 years. VFLEX technology is "state of the art" in that no other VLF detector incorporates digital interface between the coil and the controls. That results in less RFI, EMF and is JPQ. Ergonomics...... the weight of the 1500 is more than 30% heavier than the X-Terra. This will become apparent after a couple hours of swinging the coil. Speaking of coils........ the 1500 offers 5 coil choices, all at 7.2 kHz. And only
two of those coils are Imagine coils. In other words, the other three coils will not provide the imaging information such as target size or depth. This became an issue with me when I wanted to use the small coil in trashy areas. It doesn't image. (as a side note, the GTP 1350 does provide target profiling on ALL four coils it offers. even the small 4.5-inch). I guess my point is, the "sizzle" for the 1500 is their TreasureVision feature. And it doesn't work with 3 of the five available coils.
The X-70 offers 8 coil choices in 3 different frequencies. The 6-inch DD at 18.75kHz, 6-inch concentric at 7.5 kHz, 6X10 Elliptical DD at 18.75kHz, 9-inch concentric at 3kHz, 9-inch concentric at 7.5kHz, 9-inch concentric at 18.75kHz, 10-inch DD at 7.5 kHz and the 10-inch DD at 18.75kHz. And they all provide target ID, depth and sizing capabilities. The Garrett 1500 8-AA cell battery pack is capable of mounting on your belt. The 4-AA cell X-Terra is so light weight, it don't need to. The Garrett 1500 has a backlight feature, adjustible audio pitch and a belltone for coins etc. The multiple tones of the X-Terra 70, combined with the sizing capability of the Prospecting mode, do not require that I have a backlight. (I'll carry a penlight if I want to hunt at night) As well, the X-70 multiple tone mode provides 28 tones for the various notch segments. Not just a bell tone on "keepers" and another tone for "non-keepers". I will add, the X-70 does not have to be operated in multiple tone mode. Each user can chose between single tone, dual tone, triple tone, quad tone or the multiple tone mode, as they desire. Both models have notch discrimination, with zero discrimination as a baseline. But the X-Terra 70 offers true "all metal" with iron mask capability, in Prospecting mode. The 1500 incorporates ScanTrac. The X-70 offers autoGB, manual GB, Tracking and Beach Mode. Choosing whether I want to manually adjust my GB is important to me, in some of the places I hunt.
Off the top of my head, those are reasons I like the X-70, compared to the 1500. Honestly, considering the 1350 offers profiling with all coils, I'd opt for it before I'd have another 1500. But even at that, I enjoy the functionality of the X-70 more than any other detector currently available. JMHO HH Randy