My gut instinct is that the particle board is a better choice because it will stay flatter over time. Particleboard and MDF stay flatter over time because the wood movement with humidity variations phenomena is minimized compared to it's affect on solids and ply-woods.
I would not suggest just poking a big hole for the bit and a few more smaller holes for the mounting screws through particle board, MDF or ply-wood. Doing that focuses the weight load of the suspended machine on 2, 3 or 4 screws. This technique is very popular in the 'cheapest' of tables, and it invariably fails sooner than 'plate based mounting'.
Router Table Tops should be flat, but also need to be sturdy enough to support the weight of the router they are suspending. Those two considerations are what has fueled the trend of using plastic or metal mounting plates with MDF tops. The larger surface area has the stability of MDF, but the weight load of the motor is distributed across a long rectangular 'joining area' where the plate meets the MDF.
I haven't got around to crafting up a router table top myself yet, because I haven't needed to. three 'factory made' tables available for use. Two of those have steel tops, and one is MDF. Some people use aluminum tops, but something about aluminum can stain wood that crosses over it, so I avoid them.
Being as 1/32 is far less than the thickness of the average ply, if I had to level up the ply board, it wouldn't take more than 10 to 15 minutes with the right hand plane to do so. By right hand plane I mean a longer bench plane, jack plane or jointer plane.
Good luck with your 'build' and I hope you share the details of your experience with the others here.
If you haven't read through Old Chipper's thread here on building router tables yet, I would be remiss for not recommending it. With over 600 posts, it takes a while to work through it all, so the amount of insight and knowledge takes some time to absorb. It took me three days of focused effort!
Here is a quick link: Wanted! pictures of your router table!