First post for me on here, so here goes.
As far as saying that biking is a bigger strain on the cardiovascular system, it purely depends on the person. I, for example, can go for a 80-110mile bike ride and my max HR will be !75-180bpm. My roommate, a cross country runner and track runner, can go run for 13-15 miles and his max HR will be 175-180bpm. We swapped roles, and his HR was well over 200 near mile 10 of the bike, and I was only 6 miles into the run when my HR was well over 200. If your body is used to one type of cardio, it won't strain the body as much.
If you're not a runner, but haven't biked in a long time, biking will hurt more, because of thousands of factors, and vice versa.
In a perfect world, everyone would do both. I will say that personally, running kicks my cardio's butt everytime I go for a 5k run, but like I said before, it differs for everyone.
You can't compare a cyclist biking for the same amount of time a runner runs. It's apples and oranges. Remember, when you are on a bike, you have tons of mechanical advantage on your side that you don't have when running. Throwing the different gear ratios in there makes the bike even easier. What do most people do when biking up a steep hill? Shift to an easier gear, but you can't "shift" to an easier gear when running. A good rule of thumb is take your biking mileage and divide it by 5.5 to get an equivalent running distance.
Cliffsnotes: Both work your cardio system, and both depend on what you and your body is used to.