Slimming World has a 'red day - green day' system -- certain food groups on certain days. It revolves around food combining, and may not always suit your lifestyle.
Weight Watchers on the other hand is more about a point and calorie system, and so many supermarkets have their meals that you might always be able to find their snacks if you've forgotten them at home.
Both are effective in achieving weight loss, though I think Weight Watchers will be easier to stick to for the duration. Aim to loose it slowly and steadily, it's the surest way to keep it off (and think of longer term as well, what you're going to do when you come off the diet as that's what counts the most!!)
Best of luck.