So i decided to have a little look at the 1-3 vs 2-1 argument because i randomly started playing on DMM as well. This naturally meant i must pick a farming map for my early levels.

After checking the average gold per map as well as seeds and the gold from selling the 2* girls i can conclude that the difference is:

1-3 costs 3 less stamina.
2-1 gives more seeds and more exp per stamina.
Both of them have about the same gold efficiency if you count in gold from both selling girls and map gold itself.

So in the end it boils down to whether the one choosing decides that he wants less seeds and exp in favour of an amazing 1-3 more runs at early levels.