I haven't done any of those before, so I can't really say how they are. I noticed alot of people on here doing Slimming World, it looks like they have a big support group here and in real life.
I have a sweet tooth too, I love my carbs, but I love veggies and fruits too. I did the ultra low carb thing, lost weight, but didn't like the fact that I was consuming alot of fat, and not allowed to eat much veggies or fruits. I tend to eat lot of veggies.
I changed recently to GI diet, since it better fit my needs as a diet and lifestyle. They are allowed to eat alot of veggies and fruits, and even carbs, but good for you carbs. I don't even crave sweets because I get to have some fruits throughout the day.
Have a look around and read the forums about how people are doing on Weight watchers, and slimming world to see if their diet/lifestyle will fit your needs. Everyone is differen't so one diet might fit the need for one person, but not fit too well for another.
Let us know what you decide to go with, and I look forward to reading about your updates