It depends what you're looking for in a 'diet' and how much you are able to commit.
For example - I tried WW a couple of years ago and after 6 weeks I'd lost 1 1/2 stone - Brilliant! However, after that point I was getting a bit bored and hungry and it soon piled back on. On Slimming World, however, I'd only lost 1 stone 1 1/2 lbs after 6 weeks, but I was enjoying myself! (and had more to lose!) I find SW much easier to stick to and 9 months down the line, I'm still on it and almost at my 4 stone target. It's swings and roundabouts really.
I think if you need more structure, WW works best. The points structure is quite easy to follow once you get the hang of it, but you need to be careful otherwise you waste your points on junk rather than eating filling healthy food.
Slimming World is great if you go out a lot - there's so many free foods that it's easy to make healthy choices while out (no points calculating to be seen!). You still get your syns, which are just enough to not feel like you're missing out.
Personally, I love Slimming World, and I really do think that it has long term staying power, but that's just my opinion!