The loss tangent is a metric of the quantity of electrical energy which is converted to heat by a dielectric. Low loss of the material maximizes the antenna efficiency.
Here is a nice chart for losses of different dielectric materials:
Yes, Duroid is a very low loss substrate.
Suspended stripline technology is mainly used in high frequency transmission lines, due to higher Q and lower loss than standard strip or microstrip lines.
substrate gives you structural integrity. if you want to increase your efficiency you can also have a regular microstrip antenna without the back (ground conductor) but suspend it over a sheet of metal which will act as the ground.