For me it is harder to hold an erection after length work (stretching) because fatigue will result in a more difficult time holding an erection for girth. Some guys will do girth work in the morning and length work at night OR vice-versa. The thing about girth is the more you gain it the harder it is to gain length later because of the mass of your penis becomes more to gain size. Gaining girth will yield you length, although not as effective as specifically working length, you can try incorporating some of both in your routine and as you get better and learn what it is you like in the exercises better then you can focus on whichever you feel like at the time, whether it be girth or length.