Fire Emblem If Black Kingdom is a considerably harder game than White Kingdom, but not just because of a steeper AI.
Intelligent Systems project manager Masahiro Higuchi joined Nintendo Nintendo producer Hitoshi Yamagami in an interview with 4Gamer to explain this in detail.
So, aside from a higher AI difficulty, and more experience points to level up characters, Black Kingdom will have multiple win conditions to beat stages. You will still have to beat an enemy general, or a group of enemies, but you will also meet additional conditions, such as suppressing enemies, breaking through enemy lines, and defending bases. There are also maps which limit how many turns you have.
White Kingdom is actually designed to be the conventional Fire Emblem game. It's closer in line to Fire Emblem Awakening, and is also better for first time Fire Emblem players. Does this mean White Kingdom will let you repeat battles to keep your favorite characters, and Black Kingdom will not? We do not know quite yet.
We do know that White Kingdom and Black Kingdom are not mere mirrors of each other, with both games having different maps. Subsequently, playing either side will allow you to see completely different sides to each character, either as your ally, or your enemy. You essentially have to choose which side you want to have as allies, which defines the If element of the game.
Do not forget, however, there is a third path you can forge after playing both games, of which we still know little about. Intelligent Systems took advantage of the extra time they had after Fire Emblem Awakening to make literally three games.
Intelligent Systems admits Fire Emblem If's design is an experiment. If you buy the digital version, you can play as far as Chapter 5 before having to choose between the Hoshido or Nohr. You then download the side you chose, and then later, if you like, you can purchase the opposing side as DLC, which will eventually let you unlock the third story. If you decide you want to play both sides, you can choose to pass over replaying Chapters 1 to 5 if you want to.
Fire Emblem If will release in 2016 in the West for Nintendo 3DS.