I've used both. Other than the fact that some developers only come in powdered or liquid, my main interest is in which one costs me less in the long run.
For me the advantages of powder are less shipping charges and it lasts a very long time stored in it's powdered form. Liquid is fast and easy to mix up, and it seems to me that dev bought in concentrated liquid form has a longer shelf life than mixed up stock solution from powder. Although I had some Tmax liquid developer go bad on me really quick once....
I go through it pretty fast, so if I have a choice between powder and liquid in the developer I want, it always comes back to which form is cheaper per 80 square inches of film.