Had the GTI1500 a few years ago and did like the sizing feature. Have had my E-Trac for about a year and prefer it -- more powerful and versatile. If you might want the 2-box attachment, then the 2500 could be worthwhile.
I started with a 2500... liked the sizing feature but that was about all. Got the Etrac, sold the 2500 and have never looked back. I suppose there are people out there who prefer the 2500, but I'm certainly not one of them.
In my opinion the Etrac will find items deeper, is more stable, separates targets better, and identifies iron much better than the Garrett.