How is this different from Tugboat?

You can't really compare Tugboat to Drupal Forge (DF). It's like comparing Apples to Oranges. 

SimplyTest.me is a service built on Tugboat. Drupal Forge is a service built on DevPanel.

If you want to compare Tugboat to something, you can compare it to DevPanel. 

The details of that comparison, however, are beyond the scope of this FAQ, so we'll just leave it at that.