Let me start off by saying I am a member of both NCOWS and SASS, and have no ax to grind against either of them.
SASS has been around for over 20 years, I believe, and from the git-go had an aggressive marketing, advertising policy, and media exposure. What started out as a diversion for a few IPSC shooters who tried their sport with "cowboy guns" and liked it, has become a business. Their image has always been portrayed as "the fastest growing sport" and inclusive to the whole family. Right off the bat they copyrighted or trademaked the terms "Cowboy Action Shooting" and "CAS", so that technically, by using those terms any time, you would in fact be referring to SASS, whether intended or not. In other words, Cowboy Action Shooting/CAS is SASS, and SASS is Cowboy Action Shooting. They have their own merchandizing. They have their own bookstore selling videos, books and DVD's. Their Top Shooters are spokespersons and celebrities. Because they were the "first", they got the initial attention and exposure of being "new and different".
NCOWS, on the other hand, has a different mission or attitude towards the sport. I would guess that the majority of NCOWS members started out a SASS members who "wanted more", with more than a few Civil War reenactors and Buckskinners in there as well. It's what attracted me to NCOWS. I had a deep interest and love of American History, in particular the West. I have both SASS and NCOWS stickers on the back window of my truck. I'm often approached by folks asking about either. Invariably I end up talking to them about NCOWS. I usually summarize by saying " If you like re-living the Old West and its lifestyle, check out NCOWS. If you just like shooting the guns, check out SASS."
Among the uninformed, the biggest detriment to NCOWS is the percieved image that the membership is a bunch of sticklers for absolute autheticity. What we used to call in the ACW reenactment community as "stitch Nazis", those looking for any tiny anacronism or non-period-correct item or fabric, commonly referred to as "FARBs". I have yet to personally meet one of those in NCOWS.
SASS was tailored to appeal to the widest, most diverse market possible - minimal requirements (but a plethora of Match Rules), lots of classes and categories, professional and dedicated marketing, and a HUGE membership (close to 100,000, I think). I applaud their founding and promotion of Cowboy Action Shooting - more power to them.
NCOWS fills a niche within this community. A comparatively small group of dedicated, like-minded individuals who share both the love of Western History, living, and competitive shooting. You are free to choose which of those you wish to emphasize. As a smaller, closer-knit group, NCOWS does benefit from being a more "Democratic" organization, with each of our clubs/posses being represented in a Congress, not a Board of Directors. I shoot in both SASS and NCOWS events. I have more fun at an NCOWS shoot, because it's about more than just shooting, which is the feeling I get at a SASS match. I believe NCOWS will continue to grow, as more individuals find it to have "something more" than SASS. Could it grow faster if it developed a more businesslike or corporate image, and aggressively promote itself? Sure, it would grow, but at what price? NCOWS is receiving more exposure in the media, with national magazine articles, "Cowboys" TV show, and even some local news stories. It will attract those who want to be a part of it. NCOWS may never grow to be a large as SASS, but then again, do we it want to?
That's just my opinion; I could be wrong.