I think the Tour de France is a much bigger deal in terms of what is on the line and such, the only problem is that the biking as a sport has almost zero followers and anyone can train and sign up and run the Boston Marathon. Its easier to be more interested in the marathon.
The Boston Marathon is one of the oldest continuous foot races in the world. Its origin dates back to the 19th century. Every year it attracts thousands of runners who attempt to complete the 26.2 mile course. Yet this race remains secondary to the vast popularity of the great Tour de France. The Tour is beloved by all of Europe. The Boston foot race is known by only a small minority of Americans interested in footraces.
I'd never heard of the Boston marathon before the bombing... I have heard of the Tour de France at a very young age, however. There are marathons in every major city, but cycling around France isn't something you get to do every day. Armstrong also made it much more famous that before, even though he did not do it in a good way.
While the Boston Marathon is an exciting event that the best runners in the world participate in, it has none of the celebrity that the Tour de France does. Cycling produces celebrity athletes in a way that marathons have not. The interest in athletes like Lance Armstrong make the Tour de France a more popular event.
The Tour de France is more popular than the Boston Marathon, as the Tour de France has more people all over the world looking forward to it more than the Marathon. In the United States, the Boston marathon is more commonly heard of because it takes place in the states. The Tour de France has competitors come from more places to compete than the Marathon and has more fans outside of the United States.