Differences I've found between Map Pack 2 & 3:
Map Pack 2:
Great for 3 or 4 players. 2 players can be tough if you protect yourself inside the shacks outside of the main area because sometimes too many zombies come through the windows. So MP2 is good for 3/4 players.
Map Pack 3:
Good for 2 or 4 players, also very fun to try to solo. Haven't tried soloing Shi No Numa. 3 player Der Riese is much more difficult than with 2 or 4 people; it seems like you still get the same amount of zombies with 3 people that you would with 4. One bad thing about 4 player Der Riese is the zombie waves are very long, and you run out of ammo. 2 player waves are short and you don't run out of ammo so fast, so I think it's incredibly fun with 2 people.
If you're playing with mainly 2 people, get MP3. 3 people, get MP2. 4 people, either is a great buy if you don't have people who don't know what they're doing in your game...