Joanne - check out the low carb comparisons link on the right hand side of the screen.
Some of the differences I've found are that PP doesn't put caffeine off limits - (as long as it doesn't effect your weight loss). PP also starts you out at a higher carb count, I think it is 30 as opposed to Atkins' 20. But, if you followed Atkins' OWL, within a couple of weeks you would be at the PP 30 grams of carb per day. Both plans allow you to deduct fiber from carb counts.