Welcome to the site!
Honestly,
I don't think you can go wrong with either. Both are very durable units from very reputable companies which offer a similar package with comparable powerplants that are both known to be very reliable. Scag ZTR's have been around longer as JD only recently got into the big ZTR's within the past decade, but that doesn't by any means indicate they are necessarily better.
I would recommend going to both dealers and test driving each. This will definitely help narrow things down. Just like test driving a car, there will be things you like and don't like such as how the controls feel over one another, one might have more vibrations vs the other and simply the overall design which should help weigh out your decision and make one stand out as the winner more than the other.
I'm not as familiar with Scag's policy on returns, but JD offers a 30 day promise that if you are not happy with the machine, you can return it.