I have the ARD that comes with new TT. However I assumed a sunshade would be better since brighter picture. Maybe not?
A sunshade won't reduce brightness at all, so they will be brighter which may become advantageous in low light conditions (though, in those conditions you don't need a sunshade obviously). On bright days, days with lots of mirage, morning/evening shooting toward the sun or when there are reflections off other objects causing glare, etc, an ARD can offer a significantly improved view over the view with a sunshade or nothing at all.
ARDs will reduce brightness by some amount, but the amount differs depending upon the design. We did much testing before settling on a design that reduces brightness significantly less than a typical Tenebraex ARD. We did this by increasing the size of the honeycomb:
While that would reduce the effectiveness of eliminating off-axis light (and concealability) if done alone, we also make the honeycomb section longer so much of that is gained back while keeping the brighter picture. And while longer than most other ARDs, ours are shorter than the sunshades most people use--and a sunshade needs to be really, really long to approach the same effectiveness at eliminating off axis light.
Anyway, those are some of the reasons some who are not on a two-way range use an ARD over a sunshade (for those who are the advantages are pretty obvious). Unfortunately the view through many brands of ARDs can be pretty dark and depressing. We've worked hard on ours to eliminate most of that issue.