What are the primary enhancements Cities and Knights and Seafarer's add on to the original Settler's of Catan?
Opinions will always vary, but what I think one can objectively say is that Cities &amp; Knights adds more complexity and variation to the game as it adds several new dimensions: new resource types, new types of knights, new expansion cards and new 'privileges' gained as you expand your principality. So if this type of 'extra dimensions of complexity' is your notion of 'replay value', then C&amp;K would be the extension to go for of the two.

On the other hand, it does mean slightly more complex rules to introduce to new players. So if 'replay value' means being able to play with different groups of friends a few times rather than the same friends multiple times, this is something to take into account.