I have never used Weight Watcher Online because I decided to try MyFitnessPal first as it was free, and I saw no need to switch. It is a great service that has so many foods, drinks, spices, etc. in it's database. It also has tons of exercises and a baseline for calories burned, but obviously if you use a machine such as a bike, treadmill, etc. where you put in your own information such as weight you get a more accurate reading.
There is also community support and you can have friends on there that can keep you motivated. You set what information goes public. For instance, you can share just your weight loss, you can share your diary, etc. Your friends can see when you've worked out if you allow them and they can see when you've been checking in to MyFitnessPal so they are there to remind you when you need to get back on track.
It is an excellent place to store your food diary and make you honest about what you eat. You can see your weight loss progress and the ups and downs along the way, but more importantly, you can see the inches you have lost in various body parts. They have standard body areas they suggest measuring but you can add your own to the list, such as calves.
There are also tools that allow you to calculate your BMI.
I have found MyFitnessPal to be so helpful. I am injured and have been on crutches for 8 years and I was able to lose 25 lbs with the help of MyFitnessPal.
I love that it tells you how many calories you have consumed and burned and how many you have remaining so you can plan the rest of your day. Also, it helped me increase my water intake with the water counter.
I find it helpful that you can access MyFitnessPal through their app or online, both of which are very user friendly!
I highly suggest signing up today!!