For your layover, unless you're dying to get out and about in the Greater Grapevine or Irving area, you could have a nice relaxing stay at either property.
I stay at both properties, especially since the Regency has been updated.
But, if I had more than just sleeping to do, i.e. some extra stay time, I'd choose the Grand. It's newer. I agree about the awesome top-floor gym and the food options at the hotel.