You'll get the most informations out of garmin connect.

The other websites offer more ways to track what your friends does/did. So like bonz50 said, they are pretty much for "social". For example, You can follow people rides on strava and how they did on X segment compared to the others, discover who rode recently near your home, or who did the exact same ride as you. But if you upload only on strava, you won't get all the info you'd get from garming connect. So I always upload to garmin and strava (I reduced my choices to those 2)