@straightcur: Hmm, yeah, I don't really think that's a good word for it. Unsettling or disturbing perhaps, but not really "scary" in the traditional sense.
@dcoutten: Ah, I understand. From what I know, the story and maps are completely different between both versions, they really only share a number of neutral units, with most characters being exclusive to either version.
The basic battle system is, of course, the same between both, which is basically a slightly updated version of Awakening's combat. It's apparently tweaks to the mechanics that make Conquest meant more for "experienced" players. It still has the usual easy/hard/casual/etc. difficulty options, but while Birthright is structured more like Awakening, letting you grind money and gold freely by paying a small fee to initiate random battles on the map, the plot of Conquest dictates a far more structured take on things, with few opportunities to gain experience and gold outside of storyline battles (making them far more limited commodities than usual), and often setting specific victory conditions like "defend the base" or "defeat the enemy in 'x' turns" instead of the simple "rout the enemy/defeat the commander" battles, which make up most of Birthright's fights.
There are some less substantial things too, like a few exclusive classes for each campaign, but those are the main differences.
@Sound_Demon: If you want both, get the special edition. The price is the same as buying both individually, but it also comes with the $20 DLC campaign, the artbook and the carrying pouch. They're basically giving you that other stuff for free.
@dcoutten: We do know, actually, considering the game has been out for 7 months in Japan. It's just as they said, after the first 6 chapters each campaign goes in a wildly different direction and is ultimately around the same length and density as Awakening. They may be using a high-capacity cartridge for the 3-in-1 edition. They can be manufactured in different capacities.
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