Difficult to give any sensible opinion without seeing the mares or knowing about them.
One is a proven brood mare, is the other a maiden?
Does either have any temperament faults that you would rather not reproduce.
If you are deciding to breed rather than buy a youngster on the ground then maybe there is an emotional attachment there.
Honestly, start with a blue print of what you actually want in a foal/ future riding horse, height, color, build, manners etc, which mare gets you closest to your idea?
Once you see who gets you nearest to what you want, then you can search for the right stud to give you the best chance to get that.
OR radically, find the right stud first, and then decide what mare suits him.
“Never attribute to malice that which can be attributed to stupidity”