I agree that melamine will be nice and flat, and it will allow you to be making parts just that much faster. You won't get laminated plywood panels to lay as flat - we have a 200ton laminating press, and in my experience MDF or high density particleboard is always flatter in the end.
Plywood has it's place, as do HPDL / high pressure decorative laminate (Formica) and low pressure decorative laminate (melamine). Take moisture out of the picture and engineered cores (particleboards of various densities) have some very desirable properties. We had our guys build their back-benches in the shop using melamine for the drawer units, cabinets and even the countertops back in 2003 when we moved in to the "new building" because money was tight. Tens of millions of dollars of commercial woodwork later, they're still just as functional as they were almost 20 years ago. We even have one outfeed table on a regular table saw made with melamine, and although the perimter of the table is worn from sliding material off onto carts, the table is still entirely functional.
Our beam saw is 15 years old, and the solid phenolic tables are worn just as badly - but they've seen > 1,000,000 parts slide off them - the center area of the tables everything floats on an air cushion, but the edges slowly get rounded off and worn away.
If your router table is going to be a family heirloom, plan accordingly. If you just need to make some parts, Plexiglas or melamine are both great and fast ways to go.
My $0.15
Martin