Two-sided Material vs Two-sided Mesh

If I want to draw both sides of a polygon, which performs better: using a two-sided material, or using a single-sided material with the polygons flipped over?
When using two-sided materials, almost all materials will be two-sided for instancing, and when flipping polygons, the number of polygons will only double in areas where two-sidedness is needed. Models do not exceed 5k polygon count.
Which is the better option?