Running definitely harder overall. It's harder on the body (joints etc). Furthermore, Even moderate running will have you out of steam sooner than moderate cycling. Nevertheless, you can also punish yourself just as much going all out on a bike. Especially on hills.
Cycling just has a bigger range of effort from recreational up to all out cardio.
I also find that it's easier to get back to fit on a bike after a long period without sports. Running just causes aches and pains to occur sooner, and the older I get, the more likely it is that they become long lasting injuries.