Q.Which marker is better for plants?
What markers would be more beneficial for my garden – metallic or stony markers? I am puzzled to find out the solution right now. If you know which one is better in my garden, please suggest to me.
Thanks!

Metallic or stone markers will work just fine in your garden. Basically, this is a matter of personal preference. Stone markers typically lie flat on the ground like these:
http://herbsandoilshub.com/wp-content/uploads/2013/04/zzzzzzhomemade-garden-markers2.jpg
whereas metallic markers are popped in the ground and stay upright and perhaps may be more visible and hence easier to see.