You will get very mixed responses on here as there are lots of people following both plans! I think it also depends on how you eat (as in portion control or general bad food). I have done both plans so will give you my opinons on both.
I got to goal years ago on Weight Watchers then it went back on as I stopped going to meeting and met my now husband. I went back last Jan but my heart wasn't in it so I only half followed the plan. In October last year I joined Slimming World for a change.
With Slimming World you can eat plenty of free foods (things like pasta, certain meats, veg, fruit etc) and there isn't a limit on how much you eat of these foods (you have to listen to your body and when you are full). If portion control isn't an issue for you it can work very well if you have time to cook. My problem is that I don't really have time to cook all my meals from scratch and treats such as chocolate bars were high in 'syns' (you get a certain number of syns to use if you want on sauces, higher fat food etc). I did lose weigh on the plan but I eat out quite a lot so it was hard to fit in with my life.
I joined back at Weight Watchers in March time to try the new Pro Points plan. With Weight Watchers you get a certain number of daily points and 49 weekly points. Every food has a points value based on carbs, protien, fibre and fat and it is up to you how you spend your points! I use my daily points for my 3 meals a day and my weekly points on chocolate, extra yoghurts, meals out etc. There is potential to get fed up counting points and weighing things with the plan which can be a downside. I've lost 35lb on Weight Watchers and do not feel like I'm on a diet!
Don't be scared about going to meetings with either - I've found both meetings to be very friendly!