I would just concentrate of conditioning yourself first before you aim for either. Do your stretches and jelqs and after 2 or 3 months then think about focusing on one or the other if you want. In that time you might see that you are a good gainer in one particular area and as what Bnip said, you should just ride out those particular gains. I seem to have pretty good length gains so far so am just happy about that, but am doing both length and girth work so as to not overlook a part of my training. Like in weightlifting, you wouldn't just work out your upper or lower body while totally neglecting one would you? [Even though a lot of guys do, those guys with chicken legs in the gym haha]