mile for mile, running gives you a harder workout, saving time if you're in a pinch. also, mile for mile, running is much harder on the body. If time is the issue (not weather), you could do some intense intervals for a shorter time.

I know too many old people who were marathoners at a younger age that have knee problems, that I and they would attribute to marathoning. However, it's only a correlation.

AND, I don't feel like I get anywhere running at 6 mph vs cruising at 16 mph.