We can't and shouldn't automate every test case
I would choose automation over manual
-If it is repetitive like data driven
test cases that are supposed to be tested against
multiple test data.
-If the test cases are high priority test cases
-If the functionality is critical functionality
-If the test cases are too long and too difficult to execute.
-I would automate smoke test cases and also
-I would automate regression test cases as much as possible
based on the priority