The only way for someone like me to get into the NYC marathon is by lottery: Their qualifying times are significantly harder than Boston's are and yet the race is bigger--38,000 v. 25,000. My age-based qualifying time for Boston has to be under 3:30, while NYC requires a 3:10.
An initial reaction might be that the New York race is just bigger and more competitive and that is probably what they want people to think. It isn't true.
Neither organization makes it easy to crunch their numbers. But here is what I found:
NYC has about 38,000 runners and in 2008, 997 came in under 3:00 and 3,651 under 3:25. Meanwhile Boston had 25,000 runners in 2008 and about 1,282 were sub 3:00 and 5,293 were under 3:25.
Boston attracts a lot more good runners than NYC. It would be difficult to determine what percentage of the NYC runners get in through qualification v. lottery, but I would guess around 90%.
What one would have to do is search the results for each age/sex category and then find the ratio of runners who came in under the limits. For instance: I found that there were 1470 US Male entrants in 2008. Only 106 of them ran under 3:10.(which is the NYC qualifying time for men that age) That is only about 7%. Just do that for every age/sex group and then scale the percentages by the proportion each group contributes to the total...etc...
I question the wisdom of NYC setting the qualifying times so hard. Their times are not realistically obtainable for most people--so there is no incentive to work at it. Boston's are pretty hard, but most people can, with sufficient effort and a little genetic luck, qualify.