The edge you choose on your mirror will depend on a couple of things:
1. Whether or not you will see the edge of the mirror 2. The design of the frame
If the mirror edge is going to be under the rebate (i.e. hidden) your choice is between having a bevel or not having a bevel. There is no right or wrong answer, it simply depends on the overall look you'd like to achieve. If the mirror edge is going to be visible, you then have the choice of having a polished edge or a bevelled edge mirror.