It depends on the level of your girls, and what they are. 1-4 is a good early level point. 2-2 is also pretty good if they have a few more levels. Africa/Finland is good for high level, especially 6-2 and 7-2
As said, the harder the map, the more experience its nodes give. But it also depends on the rank you get for them. A harder map with B ranks will usually give less than a somewat easier map with S ranks. So it all comes down to which map can they consitently get S/A ranks in.
Using this, you can also train a very low level girl faster by putting her into a team of high-levels and throwing them into a hard map, since the others will take care of the enemies. But training many low levels together in easier maps would be more efficient overall.