what is the difference?
The two gardens are pretty similar except I find the Dallas Arboretum to be highly confusing parking wise and the entrance to the Dallas Gardens is outdoors while Fort Worth's is inside and air conditioned. Dallas also is a lot bigger but not all because of gardens some of it is just an expanse of trees and grass. It also has a lot more walking. Photography wise Fort Worth is better in my opinion. Why? my big thing is that Dallas forces photographers with equipment to be taken from 3pm-5pm. This is bad because one its Texas and that time is hot. Reason two this is a terrible time for photos. One can't take photos at that time of the day unless it's overcast or in the full shade.
So which do I suggest?
Fort Worth hands down. It's beautiful, less restrictive photography wise, they are pretty similar scenery wise and from my experience nicer people. It's worth the drive if you have to take it. Also making is a lot easier it's free and right in front of the gardens.