I have done both WW and SW over the past 15 - 20 years and found SW to be better for me. WeightWatchers is more restrictive on portion size, whereas Slimming World is more like restricting a certain food group (ie protein or carbs, but you choose which one for the day, and you can alternate if you want).
If you are a huge meat/fish eater SW will suit you.
If you are a huge carbs eater (pasta, potatoes, rice) SW will also suit you.
Just make sure you opt for the 'Original' days or 'Green' days and forget all about the new rubbish they're doing now such as 'extra easy' programme etc. Far too confusing and too similar to WW.
And before anyone says neither work 'cos I've quoted "the past 15 - 20 years" - the reason for that is I stop going to the meetings and weigh-ins when I reach my target
weight and then eventually slip back into my old habits!!