Note that I have done exactly one marathon (26.2 mile run) and one century ride (100 miles bicycling). Since I did the Santa Fe Century, I have been asked many times whether the marathon or the century was easier. So here is my amateur answer to this very easy question: The century ride. Why? It boils down to this: I enjoyed the century ride about 90% of the time. There were some difficult times, but most of it was pure fun. For the marathon, I enjoyed it maybe 60% of the time, and then the last 40% of the run was pure pain and suffering.
But there are some pros and cons:
Time (Marathon is easier)
Century: 9 hours, Marathon: 5 hours. Now imagine doing some fraction of that EVERY Saturday for a few months. So the century does not just take longer on the day of the event, every single training ride takes longer.
Expense/Equipment (Marathon wins here too)
Basically, for a marathon you need good shoes maybe even two pairs and a watch. The rest of the clothing and special food is similar in running and cycling. For cycling, you need a bicycle, preferably a nice one along with all the accessories like gloves, helmet, cyclocomputer, repair kit, shoes, etc... Note that if you travel a lot during the training season, it is more difficult to bring all this cycling gear with you than running gear.
Pain (no clear winner)
This probably depends on the individual. Running hurts joints and can injure specific muscles. I now have plantar fasciitis which prevents me from running but I don't blame the marathon because the pain started 3 years later. Also, I found chafing to be a bigger problem running than cycling. Cycling could be quite painful on the hands and butt. I also fell over a couple of times on the bike, but with no injuries. I've never fallen while running. So there is more potential for pain from crashing a bike.
Recovery (Century wins)
After a long training run, I'd be done for the day and would hardly be able to eat. After the marathon I took an ice bath to speed recovery and the next day I could walk slowly but stairs were somewhat challenging. After cycling, I'd simply go back to life as usual except that I couldn't go running the next day. Cycling never bothered my stomach the way running did.
Flying downhill at 30+ mph, yippeee! No such freebies in running - it's all work uphill and down. This also makes cycling less monotonous than running. Running literally is "one foot in front of the other", hour after hour. Whereas cycling feels quite different whether you're heading up, down, or around squiggly roads. There's also the pace line to keep you entertained during flat, straight sections. On the flip side, I find it easier to talk to my buddies while running than while cycling.
There are plenty of other issues that divide marathoning and century riding, such as the fact that weather can be more of a problem to cyclists than runners. But then cycling can be a more useful form of transportation than running/walking (you get better distances) so it may be easier to incorporate cycling into your regular day.
Personally, I will likely never do another marathon but I'm already thinking about which century ride to do next.