I've come around the fact that this is like comparing apples and oranges.
I am a 'fragile' runner, unfortunately. If I go much above 40 mpw, I get injured and body breaks down. I love running, was a runner first - and I agree with Armstrong that running a marathon is probably the hardest thing I have done. But that is only because of the 'pounding'/impact nature of running. It isn't like it necessarily taxes your cardiovascular system that much more.
Biking, I can crank out 40 to 50 mile rides a day - really pushing too. Climbing 4000ft during those rides and on flats averaging over 20 mph. They are 'hard' rides, they just don't break me down, even thought I am tired by the end.
I'm actually coming around to preferring biking. I've managed to get in better shape biking because I can tax my cardio system more. With running, I am limited to 40mpw and even then most of it has to be easy mileage and I'll be walking around with some sort of annoying little injury that doesn't stop me from training but I have to pay attention to it. For those who can handle the impact of running - I am jealous. The flip side is biking is a pain in the ass in terms of time commitment.
Basically, I don't know how you compare - one beats the shit out of your legs due to impact so it limits how much you can do over time; the other you can really tax your cardio over longer and harder periods of time