Slimming world all the way! I've been to both, Slimming World makes more sense, you can eat more but you definitely do lose weight, plus there's no silly weighing and measuring or calorie counting. The group I used to go to was very friendly and they had nice little incentives and lots of recipe testing which was fun!
Here are the principles of Slimming World:
As part of its food optimising system, Slimming World has designed two eating plans - Green Days and Original Days (often referred to as Red Days) On Green Days you can pick from a huge number of free foods including pasta, rice, potatoes, fruit, veg, beans and pulses. These can be mixed and matched with 'healthy extras' such as lean meat or fish.
Original Days allow for a higher protein consumption and free foods include lean meat, fish and most fruit and vegetables. On top of this, other foods are given a 'syn' (synergy) rating and, depending on your goals, you are allowed a certain number of syns a day.
It's really easy to stick to as you can eat the things you fancy in moderation, and if you're doing it right you're never hungry. When I used to go I lost 2 stone in 3 months, but then Christmas happened and I somehow ended up not going back as I was embarassed I'd put on some weight. Wish now I hadn't been so daft...
Not much to say about Weight Watchers. My experience was an overcrowded class of hungry people, and measuring out teeny tiny lumps of cheese, plus they advocated buying their own brand (teeny tiny) frozen ready meals which I don't think is good practice as low fat or not, ready meals are notorious for being packed with salt and preservatives!