The only reason to put a liftback on a car is so you can fit bigger things in it. If you are not gonna do that, why not just get a standard deck-lid trunk? If you want one for the space, why not get the most space? The car is going to get heavier because of the lift back anyway, so why not take full advantage?
BMW will sell you three liftbacks in the same segment now. Which one should you get? If your looking for the most cargo room, the answer becomes obvious. You might not care about that, but then what reason do you have to get the other two? Why not just get the standard decklid?