It depends on what and how much you are going to put on them. A thicker hardwood will hold a a ton. Melamine will sag. And probably not an issue, but if the melamine or plywood get wet that can cause other issues.

I would use 5/4 Poplar. Any decent lumber supplier can make the shelves in the size you need. The cost would should be comparable to high quality ply. And less labor. Google "hardwood supplier" to find one near you.

No worries about finishing the edges, plus Poplar takes paint great.