I have two ZTRs, a Scag Tiger Cub with a collection system, and dethatcher and a Hustler X-ONE 60" rear discharge. For what a ZTRs do, which is mowing grass, they do it well, but, that is about all they can do!!!
My GT does logging, hauling, tilling, disc harrowing, grading, snow blowing, plowing, moving trailers, towing dead vehicles, mowing, mowing with collection, dethatching although slower than a ZTR. My 1989 Ingersoll 4018 GT is on it's 3rd engine with about 3,000 hours total on the machine. I use it as a backup for my ZTRs which has happened a few times much to my dismay!!! Moving a non functional ZTR is not fun!!!
I will never be without a GT and a ZTR.