I understand that there are many ways to design a solution. When we should use a

When we should use an asynchronous vs synchronous processes, is explained with an example in below link.
If you have missed this forum.appian.com/.../Using_a_Sub-Process.html
The important point here is: We have to use synchronous sub process, when we need sub process activity (or sub process variables) in parent process.
If parent process activity and sub process activity independent of each other, then both these processes follow asynchronous relation.

Further to above, a couple of examples: a common use case where synchronous process behavior may be desirable is Approvals where without an approval decision by assigned user the process cannot progress onwards. A common use case where asynchronous process behavior may be desirable is Sending Emails as in this case the progress of the process doesn't have to be designed to wait until the email is sent.

Another aspect to watch out for is when launching many sub-processes at a time from with in a parent process. As the Sub-Process instances start on the same execution engine as the parent, you might want to reduce the load on same engine by launching these sub-processes via sending messages instead of using sub-process node. Below documentation section contains some information on how to do it. One highlight from this section is "Avoid using sub-process activities to start processes based on query rules when each new item in a data store starts a new process. Use process-to-process messaging instead."

forum.appian.com/.../Messaging_Best_Practices.html

Thank you guys for all of your valuable input.