Ansible is the command line tool that does most of the work while Ansible Tower is a web application that provides a REST API, RBAC, secure credential storage, and a scheduler (among other things) on top of Ansible the command line tool. You can’t have Ansible Tower without Ansible, so comparing them directly is not quite accurate.
Ansible Tower’s upstream opensource project is called AWX. If you don’t want to pay for support you can always go AWX instead of Ansible Tower. https://github.com/ansible/awx