I did weight watchers on line breifly a while ago and would say the difference is there is less weighing and measuring in slimming world. Lots of free food to fill you up and then a few bits and pieces to measure. Lots of fruit and veg to be eaten as well as lean meat, eggs, fat free yogurt and pasta/potato/rice etc. Then you can have a little bit of bread, milk and cheese and a little bit of anything else you fancy.
for me the downside of sw is that it is much harder to have preprepared food. If you like cooking it is much easier as you are not counting points of all your ingredients, but if you like putting a ready meal in the microwave it is more difficult as even low point ready meals are usually high syn.\
I have heard though that weight watchers does now have free food as well so not sure about the new ww plan. Good luck