Both, and very good at both.
You want DD upgrade your skills... you will see DD, you want tank again upgrade your skills, those like martial focus etc. Get a shaman to be your healer, Wards ROCK, and brawlers are the best damnned tanks around.
For DD or tank would also help to have up to date equipment
also rember to use your stances staggering and brawlers when tanking thundering and fiery when DD, staggering also as it does add AGI, jus dont do it on the MT... as that defeats purpose of a main tank.