Either will help you get to where you want. However, you may need to bike at a higher intensity than you would running. On the other hand, biking is much better for your knees, and could possibly build your legs muscles more than running can. What I personally would do, is switch it up. Three days a week go biking instead. Try to find some slight hills to go up, use resistance, and go a lot farther. For me, I am much more likely to keep to a routine when I switch it up.