Jack and Oscar are more likely to contain action, which makes Rob and Greg likely to contain more humor. In my experience, humor ic conveyed almost as easily through text as it is through image, while action is usually better when you can see it. Considdering that my ratio of correct predictions on what will happen next in a webcomic to predictions on what will happen next in a webcomic is very poor, I am confident that it would be a better idea to stay with 1: Rob and Greg.