I'm planning on building my own platform bed. I've done extensive research but could not find the answer to my question. Is it better to build the platform with a soild top (i.e. plywood) vs a plank type top. This is the top where the mattress would sit on. Please give me your opinions and the reasons. TIA

Unless the plywood is supported well at regular intervals it will start to sag down, so is not a good choice. I would suggest pressboard as it is stronger and will withstand the weight of the mattress and a body for quite a while without sagging.