Both of you using MDF for shelves - I'd use ply. Less saggy, less heavy, less horrible dust. Takes paint just as well - maybe better on the end grain once sanded.
What would be your method of fixing ply shelves to the sides of a bookshelf. The MDF shelves we have a just screwed straight through from the outside. I imagine that would split ply.
Both of you using MDF for shelves - I'd use ply. Less saggy, less heavy, less horrible dust. Takes paint just as well - maybe better on the end grain once sanded.