No, this is not going to be one those my diet is better than your diet posts, accompanied by flaming comments from the true believers of either plan. Sorry to disappoint. Rather this is about why a person would choose one plan over another, and be successful. Each plan has its success who are die hard adherents and turn it from a way of eating into a religion. Each plan has had converts from the other plan, and each plan has its failures. In a short one word answer, its about compliance. Which eating plan are you most likely too stick to over a long period of time, and why?
I am going to admit up front that my own personal preference is for the South Beach plan. This is based on my attempts at very low carb dieting, which I hated. With in a couple of days of starting phase one of SB, I always start to feel "better". Even after several days of induction, I can't shake the feeling of death warmed over. I never seem to get to that happy ketosis place. Even accounting for the people like me who don't do well with induction, there seem to be other components which make one successful on one plan versus the other.
While in each eating plan, both red meat and seafood are allowed, even encouraged, the Atkins plan seems to draw the meat eaters, while the seafood folks gravitate to SB. I am sure there are exceptions to this observation, but based on my observations both on and off line this seems to be a general distinction between the groups. Using myself as an example, if I had to eat Fish or beef every day for a month for dinner, I would choose the fish. I like beef, but if I eat it once a week that's enough. I cannot, however, say I have met (in real or cyber space) a practitioner of Atkins who would make the same choice, or that has the same preferences. Maybe they exist, but I haven't met one.
A second area where preferences seem to differ is the choice between olive oil and butter. I'll admit that I think real butter from grass fed cows has a place in every kitchen. I also think that the fear of saturated fat is unwarranted. Most SBer's I know use a bit of real butter now and again. Many Atkinser's use olive oil as well. Perhaps because the SB book encourages olive oil, and the Atkins book doesn't discourage butter that this is a side effect of the plan, but I also think that personal preference plays into this as well. Given the amount of veggies both plans encourage you to eat, a preference for olive oil or butter may be a factor in deciding which plan is best for you. While its true that you can use olive oil on Atkins and one the butter type spreads on SB, if you start looking at the recipes and menus associated with the plans, the trend becomes clear.
There are certainly other points of distinction between the diets, that may be make or break issues for some. Some will prefer to count carbs, while other will prefer to follow the rules and food lists of SB. Some people won't like either diet. With all of the noise lately about low carb vs low fat vs the Mediterranean diet, I find that the South Beach plan provides a livable mix of the best parts of a reduced carb and Mediterranean type diet. Success on any plan is about the commitment to change the way you eat permanently, whether you ultimately end up choosing South Beach, Atkins, or even the Ornish plan,