You really cant compare the two because the GTI 1500 is a discrimination machine, the DFX has all metal mode, the GTI 1500 does not, so it will not detect as deep. However the GTI 2500 will be better comparison to the DFX as it has the all metal mode and will punch deep. It images, show depth/size, easier to use than DFX. My money is on the 2500.
I've read many posts over the years either talking good or bad about all-metal mode, yet in 10 years of deteting, and hunting with many people, I've yet to see a single person use it for even 5 minutes! I also don't recall reading too many posts or people using it, nor do I see much of an advantage, just something that wastes time.
The 1500 I thought was ok but there are some things I don't like about it. An XLT or DFX never had my interest because I've always had the impression there are too many settings and that can keep a person guessing to what works best. Regardless, once I had a few months on my explorer my detector debating was over.