Hi BYB! I've done both, and I think Atkins is more user friendly. The do's and don't list is more simple - DO eat meat, fish, non-starchy veggies, and cheese, DON'T eat bread, pasta, sugar, or fruits high in sugar. It's been a while, but I remember South Beach being more restrictive with the fat intake, and it would allow starchier veggies and more grains.
Weight loss was much slower for me on South Beach. I once lost 60 lbs on Atkins, and I did the same as you - gained it all back plus some! I think Atkins is great for getting the weight off, but when you lose the weight, it's important to learn how to eat again. Though Atkins does have a good maintenance plan, some folks are just dying for an Oreo (or other sweet), and then one turns into a pack! The carb monster wakes up in us, and we are hooked all over again!
This time, I plan on doing Weight Watchers for the last 20 - 25 lbs I have to lose, but I'll keep Atkins at the core. This way, I'll be able to have the Oreos, cheesecake, etc., but in moderation. At the end of the day, that's what it's all about. In my opinion, Weight Watchers is an excellent plan, when it comes to maintenance, because it allows you to give into the "bad stuff" from time-to-time.
Words of Advice - If you are sticking with Atkins as your weight loss plan, aside from the shakes, try to stick with "real food." Most of their bars and treats have a lot of sugar alcohols, and I've noticed that I lose less weight when eating them. I've also given up on my beloved Splenda. :-( I switched to liquid stevia, instead. The single-serving sweetners all have at least 1 carb. The carb comes from the bulking agent that is used. I would use at least 3 Splenda packets in my tea, and I can easily have about 3 cups a tea in a day.