You should NOT do such alignment anyway, because if you rename "some_function" to "another_function", then you will lose your formatting.

Instead, format arguments in a separate block:

    some_function(
        arg1, arg2, arg3, arg4,
        arg5, arg6);

When arguments are aligned in a separate block, both spaces and tabs work fine.

My own preference is tabs, because of less visual noise in code diff [review].