The short answer is packet tracer is a IOS training tool while the OPNET solutions are for network planning, performance management and R&D. So the solution you choose would depend on the goals of your simulation.
I have not personally used packet tracer but have come across similar solutions before where you get the feel of interacting with the actual router and network albeit for just the control plane aspects. As far as I know such solutions are (a) single vendor and (b) have no or very limited ability to show performance of data plane traffic. On the other hand I have a good bit of experience with OPNET products which are full blown simulation engines that can simulate both the routing and data plane sides to enable complete network performance planning and management.
Some OPNET solutions also allow you to change the protocol behavior making it suited for protocol R&D.