Focusing on Length

Some people say that you should focus on length first and girth can come later. Is it a better idea to do that? Do you gain better because your only focusing on stretching and gaining length?

Who says that? You can train for both in one session. It's usually best to do your girth work before your length work. Intense stretching can tire out the penis making a proper semi erection immediately afterwards difficult for girth work. Don't forget your stamina work and cool downs afterwards to drive blood into the penis. This will speed recovery and keep the tissues "pumped".