Izeloz, in my experience the flat bottom cans aren't convenient for use with the Caldera Cone as the bottom is too wide to fit through the top opening in the cone easily. You can work around it by opening the cone slightly, but it isn't convenient.
I would expect the round bottom to be more efficient since it is thinner and has a greater surface area then the flat bottom. I have tested the two cans against each other (not in the Caldera set up, though) and my round bottom can was more efficient. It wasn't necessarily a fair test, however, as my round bottom can has seen more use and is somewhat blackened.
In any case, the difference in efficiency was very small and wouldn't affect my choice of can. I'd get whichever one is more suitable for you, which basically comes down to the type of pot support you use, in my opinion.
BTW, I've had no problem with the ridgeline on my can being too small in the3 Caldera Cone. Maybe you were unlucky?