What unique features do Cities and Knights and Seafarer's expansions add to the original game? How do they make the game more playable/interesting?
Opinions will always vary, but what I think one can objectively say is that Cities & 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&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.
The tradeoffs are often the usual case... with more complexity, there's a higher learning curve, and all other factors being equal, the game takes longer. However, you do get more depth, strategies, and things to do.
Features vary between publisher (e.g. Kosmos vs. Mayfair) and edition (3rd edition and 4th edition Seafarers have some maps that are NOT in the other), but overall....
Seafarers adds new scenarios, or otherwise known as map layouts. They don't just slightly rearrange some tiles around. Some have 4 islands, other have a "fog of war" where you flip tiles over as you discover them. There's also a Gold resource when you get, you immediately get a resource of your choice. Special conditions, like building on a foreign island gets you a bonus +1 VP. You can build ships to get a passageway over beach or water edges of tiles. The pirate ship can be used to halt building of ships, and steal a card like the robber
C&K replaces the Development cards with Progress cards. They come in 3 different decks, and you can't buy them. You need to earn them through die rolls and tech. You can physically place knights on the board to do some minor shenanigans (like chasing away a opponent robber, or opponent's knight that's blocking a settlement spot), but they primarily are to defend against the Barbarian. There's a separate track for the Barbarian, and when it occurs, they either reduce a city to a settlement, or if the collective strength of every is enough, rewards may be doled out.
Metropolis is like in-breadth Largest Army that's 3-wide (there are 3 different ones of them), and there are also a new resource called Commodities that correspond with the traditional resources (Wood and paper, ore and coin, sheep and cloth). The commodities are used to upgrade your tech calendar.