Sengoku and CK2 are so different from a gameplay perspective that I'm baffled that people consider them similar. There are similar concepts (vassals, levies...), but Sengoku is basically all about war and conquest, and CK2 is much more about diplomacy, though of course with a good dose of war thrown in. For instance, in Sengoku, you pay for levies on your own dime, but in CK2, your vassals are paying for them. This sounds like a fairly minor difference, but in fact it changes the entire game. In Sengoku you can go around fighting wars nonstop, but in CK2 your vassals get cheesed off if you keep their levies raised too long, and they'll only give you as many troops as they want to let you have, too. Sengoku also gives you full control over a territory as soon as you capture it, whereas CK2 uses the warscore system much like the EU games, which greatly affects the dynamics of war and generally makes it much easier to conquer the enemy.
I basically consider Sengoku a wargame with some character interaction thrown in, whereas CK2 is a character game with some war thrown in.
Sengoku and CK2 are similar from an engine/UI perspective, but the real meat of a game is in how it plays.
