Hi Laila,
"Configurable Subsystem" was introduced earlier than "Variant Subsystem". As per my understanding, "Configurable Subsystem" replaces itself with the active choice completely, but "Variant Subsystem" doesn't. However, I wouldn't recommend using "Configurable Subsystem" because it will be deprecated in future versions. Here's the documentation link for the same: https://www.mathworks.com/help/simulink/slref/configurablesubsystem.html
I also have to point out that "Variant Subsystem" has undergone several improvements in the last few years and you might not face these memory issues with the latest release. You can use the "Update Diagram" option for the "Variant Activation Time" in the dialog for Variant Subsystem. This will ensure that only the active variants get compiled and executed. Here's the documentation link for the same: https://www.mathworks.com/help/simulink/slref/variantsubsystemvariantmodelvariantassemblysubsystem.html
I hope this helps.
