When Vinny literally takes the wheel in the ATS QL. This is amazing.
Basically,
Birthright is structured more like Awakening and to a lesser extent, Fire Emblem 8: Sacred Stones, where you have a map and can visit different locations and essentially grind older chapter's maps for as much experience as you want.
Conquest is structured like older FE games (except 8) where it's very linear where you can't travel back to older chapters and grind for experience. You have a set number of experience points you can gain and have to make do with that.
According to people who've played it, Conquest seems to have the tighter map design of the two stories and is seen as the better game.