A mule has nothing holding the heel, which means the whole shoe is retained by the vamp across the top of the foot and by friction underneath. That has consequences. A mule that is even slightly too large will slap and slide and cannot be corrected with an insole, while a mule that is slightly too small will rub across the top of the foot within an hour. Sizing down is the usual advice and it is right for a leather mule that will give, and wrong for a synthetic one that will not. The vamp position is the other variable, since a vamp covering more of the foot holds it more securely and shortens the leg visually, while a low vamp exposes more foot, lengthens the line and grips less. That is a genuine trade off rather than a style preference.

A flat mule or slide is the summer workhorse and the more practical of the two, and the version with any kind of contoured sole is worth seeking out since a completely flat leather sole offers no support over a day. A heeled mule looks considerably sharper and is significantly harder to walk in, because the foot has to lift the shoe with each step and a higher heel makes that harder. A block or wedge heel is far more manageable than a stiletto in a backless shoe for exactly this reason. Neither version handles stairs or a long walk well, so a mule is a shoe for a day of short distances. Grip matters too, since a smooth leather sole on a polished floor in a backless shoe is genuinely unsafe.

We wear flat mules constantly and heeled ones rarely, and the honest reason is the walking. If a heeled mule is the right shoe for an outfit, a low block heel with a wider vamp is the version that survives an evening. On material we prefer leather over any synthetic for a backless shoe, because the fit will improve with wear rather than staying wrong. Adding a thin rubber sole to a leather mule solves the grip problem for very little money. If you take one thing away, buy the size that grips today rather than the size you hope will stretch, since a mule that slides cannot be fixed after the fact.