I use 4000 coins at upgrade cost which is girl level 58.
So for girls level 58 stage 2-1 is 50% better than 12-3.
For all girls under level 58 stage 2-1 is more then 50% better.
For girls level 70 stage 2-1 might just be 30% better 12-3.
I don't have any girls passed level 70 so I really don't know about those.
I am free player so I don't think I really have to worry about getting girls to level higher the 70 anytime soon.