Definitely go Powertech - you get to set people on fire, then punch their head off. What more could you ask for?
This ^ i chose mercenary at first and regretted it so i re rolled a powertech, never looked back. Why? Mercenarys fighting style is very stand still and shoot and spam missiles whereas powertech are much more fun to play and have a much better survivability but still very good damage also their abilities are awesome! IMO go Powertech.