I can't speak for the entire Greek system, but I know that at my alma mater (University of Colorado-Boulder), by the end of the next school year, there will be twice as many fraternities as there are sororities (18 and 9, respectively). Despite this, during last year's recruitment (there were only 14 fraternities at the time), the men only had about 500 potential recruits go through, whereas the sororities had about 900. Both of these were the campus' largest recruitment for either group of organizations. The sororities completed the process with around 1800 members total (including the new members, though not all 900) and the fraternities had about 1100 total (including new members, but not all 500).
I think a great deal of the disparity might first come from a lack of publicity, but that isn't necessarily the fault of our fraternities, per se as the fraternities' situation is a unique one (
http://www.dailycamera.com/ci_12960503). Secondly, though, I think a lot of it is because a gentleman is not required to attend all houses or go through some sort of party system; likewise, the fraternities do not guarantee a bid (following full completion of a preference card) or have any sort of quota system (the largest houses have around one hundred twenty members, while the smaller houses have somewhere between twenty and fifty members).
I am not necessarily sure which recruitment is easier--I think they both have their own specific hardships/peculiarities. It is definitely true that the sororities tend to have larger numbers of PNMs going through and larger numbers of them joining houses, but I do not know how a structured fraternity recruitment would compare. Are the numbers more comparable at universities that have a more structured fraternity recruitment?