This question comes up quite often. I think you need to set clear goals first and then the answer will become apparent after you do. For instance I used to switch back and forth between being strong and being & looking lean. Where I ended up was being heavy (28%bf) and not all that strong. Conflicting goals there.
So first define the goal! Then create a two headed plan. Diet and exercise to get you there. Right it down, Print it up, talk about it all the time. Makes it Real. Switch up your training often, learn about periodization (http://www.testosterone.net/html/body_129per.html) Let nothing deter you from it. Not your wife, not work, not you own lack of knowledge. Set your eye's on the goal and don't look left or right but always forward.
Track everything, food log, training log, measurements both visual and recorded (tape measure). You need this because you can NOT figure out where the heck your going with out knowing where the hell you have been. Oh and please do not make excuses, no whining about being a hard gainer or having a slow metabolism. When you get to a challenge, learn how to attack it with out letting it defeat you. Everyone faces challenges! You can push through, you just have got to want it bad enough.
Sorry if this came out like a rah-rah speech or the diarrhea of the mouth. I have set my sights on 10%bf and been hard at it for over a year. I figure I got another six or seven months to get there, so a lot of this was poking a finger in my own chest to keep me going!