Hi William,
Speedgoat target machines are expressly designed to work with Simulink Real-Time. Similarly, only Speedgoat hardware is supported by Simulink Real-Time starting from R2018a, as indicated in the corresponding release notes. In other words, if you'd like to use Simulink Real-Time, then you will need Speedgoat hardware.
There is a number of differences between the workflows you mention, but the most important one is, in my opinion, the flexibility and versatility offered by both Simulink Real-Time and Speedgoat. For example, you can extend the capabilities of your Speedgoat target machine by adding individual I/O modules should your project require to use an additional protocol that was not planned initially.
I hope this is helpful!
Dimitri