I have a dwarven barbarian, and I was wondering if I should get the Heavy armor proficiency as a feat?
Would it cancel any of my barb abilities? I could use a higher AC when dueling, don't know if there is a trade off, or if it would be worth it.

The penality for using heavy armor are skill checks with some skills as are: Hide, parry, listen,... plenty others as well :(

As for being worth getting i say you chose what you think is cooler ;) (Fashion over usefulness :D :D ) If you like your barbie to wear heavy armor...So be it ;) The game is quite easy so you wont have more problems choosing medium one :)