I voted "other," but I think riding may be harder for all the reasons stated, plus a couple of personal ones. I am a new runner and I've been riding for 9 years. Because I'm in pretty good aerobic shape, running is more of an issue of leg strength and avoiding injury for me. I am not fast and don't want to be. Well, I've improved my speed, but every little injury/illness sets me back, as opposed to cycling. I have to run/ride *up* a 6-15% grade on my street every time I come home. Usually, on a run, that's where my cool down begins, so I walk. However, I have run up it and actually felt better than I thought I would. It's slow, but I am running. And I think cycling is what helps me here. I live in a hilly area and routinely climb grades that are tough. I spin on all climbs, sometimes very slowly, but I get up. The only time I have walked was last summer on a hill that was a mile or more of 18-22% grades. In retrospect, I could have done it, but my friend walked and once i saw that, it did me in.
As others have stated, it's easy to slow down and walk up a hill when you are running. Even though I am only an average rider, I have confidence on hills, since that is all I've ever known. I haven't run up anything steeper than my street, so no 20% grades to speak about for running. There's some mechanical advantage to the bike, but as I said, you can always walk on a run.