Compare these two images, which were drawn with the same identical brush stroke:

The difference? The one on the left was done with Mirror mode enabled. The one to the right had Symmetry mode enabled instead.

Actually, we drew the one on the right first, in Symmetry mode. Then we applied Mirror mode to it, and caused the left half of it to be replaced with the mirror image. Mirror mode can be used on-the-fly with your brush strokes, or post-mortem.

Mirror mode gives you a perfect mirror image copy of the other half. Look at the area in the middle, the left and right halves are totally mirror copies of eachother. You draw on the right side of the symmetry line. The left side will be mirrored pixel for pixel, everytime that something significant has been done to the image: perhaps you paint, perhaps however you use a filter. Anything goes.

By contrast, the symmetry mode picture is not perfectly symmetrical. The reason? because we didn't want it to have the predictable look of robotic perfection. So we took a different approach for this mode. To simplify it: We're alternating the brush stamps between left and right side. When the Step distance is small enough that the individual impressions of the brush image overlap and fuse together, you can't tell much that it's doing so. However if you increase the Step distance, you can tell that something interesting is going on. Here;s an example with a high Step value:

Here's another image to show it. This one also shows two other artifacts:

• If you use image-based brushes (Custom brushes) that carry text, you may not be able to red it in the mirror copy. But you will continue to be able to read it in symmetry mode.
• If you enable Post-FX on the brush, specifically Drop shadow, the shadow will appear to head in the 'other' direction in mirror mode, whereas it'll continue to appear going the same way in symmetry mode.

So when do you use Mirror? when it needs to be a mirror image, or if the image is already there, or you're going to do work with filters that you want to have mirrored.

And when to use symmetry mode? When you don't need perfection, when in fact you'd rather not have it look like the computer did the work for you. Example: Face painting...

Related:

Symmetrycal Painting

Kaleidoscope mode