Its been a while since I researched this, but from memory a heel wedge works more quad, and flat works more glut. Heels up makes it feel easier for a lot of people to hit parallel. The wedge moves center of gravity forward a bit, so that the upper body can stay more upright during the lift.
Interesting point from Ruth about foot support, but I never noticed any difference between using a board or plate compared to shoes with heels.
I think either way is fine. People have different body dimensions and might feel more comfotrable with different foot positions.
In the 80s when I was powerlifting I remember the trend switched from heels to flats, with the argument that you have more potential with flats by using more back and gluts. But I notice that a lot of powerlifters are using shoes with heels these days.