Is duplicating an object with Shift-D + checking "Link" the same as duplicating with Alt+D, or different? I have always used the latter.
That is exactly the same. With SHIFT+D you get the same pop-up menu as with ALT+D, but with ALT+D the box Linked is checked and with SHIFT+D it is unchecked, so you can change one action into the other without any problem.