The quality of bone alone makes this the better horse. She looks similar to the first mare in the shape of her hip and chest, but this one looks more substantial, more athletic, and more sound.

Pros:
Cute feminine face and moderate throatlatch
Neck comes in well on top, adequate on bottom
More sloping shoulder and better set humerus
Smooth, strong topline
Short back/loin, close coupled
Open angles in HQ, good bone proportions
Heavier boned w/o being coarse
Basically straight legs from front/side/back (all 4)
More consistent muscle and balance

Cons:
Neck looks heavy, thick, short
Has that pigeon breasted look
Withers look high vs croup
Shallower heartgirth- belly is lower than bottom of sternum
Lacks fullness over the top of the croup
Not as low and forward in the stifle as the other mare

I think the other mare has a better hindquarter, but this mare has a better coupling, not to mention her overall more compact, athletic and sound appearance.

PS I wrote this before looking at other people's responses.... sounds like a unanimous decision!