Anyone have any comments on whether mirroring geometry or combining geometry is a better method. Are there any pros or cons to each.

I recently tried to combine two objects and they dissappeared. I then checked my normals (all okay), my verts (all merged), and geometry. I couldn't find anything wrong. So I decided to try out the Mirror Geometry and it worked perfectly. I also noticed that it seems to automatically combine objects at the same time. So now, I'm considering to always use the Mirroe Geom when I can. Just seems quicker and easier. Any comments on this?