The quick and easy answer is niether. Both Melamine and straight ply will not hold up to the urates of either species, by themselves. However, with an additional protection such as useing a laminate, or perhaps PVCX for a floor and up the walls, and using a protective finish on the ply. You can get the additional protection required. Even vinyl flooring over a painted interior is better than ply or melamine by itself.
So the long answer is you need something else to protect the interior than just melamine or ply alone. Which one, since there are many options is really up to you, and totally based on the material you decide apon. Basically you really need to decide what you want the cage to look like and what materials you have the ability and tools to work with.
As far as your dimensions I think you should re-think them, based on animal size. Basically an 8' cage is a good size but impossible to move around door ways in and out of buildings can be difficult. 8' cages are easy to construct because sheet material comes in 8' lengths. However, it also comes in 5' lengths or 60", not from the big Home Improvement stores, you need to find a cabinent supplier or a wood supplier that sells the shorter stuff.
With the ability to get shorter ply you can economically construct 2 cages both 5' in length. You can still do it with the 8' material but there will be a bit of waste. If you build the 2 5' cages you can bolt them together and have one 10' cage much better than a single 8' cage and far easier to move. You could even latter down the road build a third cage to add to the origional two.
IMO the two cages is a better setup but it is your cage.
Good Luck
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Jeremy
"I am become death, the destroyer of worlds" July 16, 1945 Robert Oppenheimer
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