I've done both, so I will try to be unbiased!
I did WW on and off for years, and always found it OK. Since they changed to the Pro-points thing, I found I was always hungry - hence why I swapped to Slimming World, despite always saying it wouldn't suit me!
I averaged a 1lb weight loss a week on WW, but on Slimming World I average 1.5lb a week. I am also never hungry (although thats not to say I don't get cravings!)
I think there are 2 considerations to make before starting Slimming world - do you eat a lot of bread, and do you like conveinience foods? If the answer to those questions is yess, you may find it hard to adjust. Although you can have a couple of slices of brown bread as a healthy extra on Slimming World, anything over that is really high in syns! Not a problem to me, and I feel less bloated without bread, but some may struggle!
I always relied heavily on packaged foods - jars of sauce, pizza's, chicken in breadcrumbs etc, and these are pretty much out on SW unless you want to use your syns on them. I did find it a struggle to cook so much more from scratch on SW, but it's changed the way I cook now, and has taught me much healthier eating habits.
No diet is perfect (if it were we'd all be size 10's!!) - you just have to find the one that suits you.