What kind of Chimney Liner do I really need?

"tickator" is right. Go with the guy that told you to use the same size liner. The other guy is just trying to sell you something you don't need and trying to make his job easier in the process (the 5" will drop down the chimney easier). As for your old one working fine, i would only say, steel rusts (even stainless 'rots') and you do not want a liner leaking carbon monoxide into your home. Play it safe, and replace it.