This is a professional lawn mowing forum. Almost everyone will say ZTR, even though most of them have never owned or even used a good garden tractor. I have. I still do. The Simplicity would be my choice if you go with a GT in your price range, but you may want to look at the JD X500's as well. I'm sure there at least one model in, or close to your price range.
The thing you need to ask yourself though is-
what do I want to be able to do with it? Mow only? Any gardening type work? Year around use for all of the above, plus snow removal?
If the answer is either of the last two, then the GT is the only smart choice. Do you like to mow, and don't have to get done in the fastest time possible? The GT is the way to go. If you hate mowing and consider it a chore, the ZTR is the way to go since it is quicker (though not nearly as much as many guys would like to imagine). The ZTR will leave ruts and divots that no GT will. Read through the posts on this site. Even pros complain about the ruts and divots etc left by constant use of a Z, and I'm not talking about permanent ruts from the weight, but rather bare ground ruts from traveling over the same place due to ground cover, beds etc, where the heavier Z will eventually leave permanent bald ruts from each rear tire.
A GT is easier to operate efficiently than a Z unless you use a Z every day. Mowing one time a week doesn't give you enough time to become proficient with the Z.
The Z's have their place for sure, but I think more HO's are better off with a machine that anyone in the family can get on and do a good job with, without any previous experience.