Fine Woodworking Magzine recently published an article about painted
cabinets. The main point of the article was that if you are painting
the finished product, there is no need to use anything but MDF. It
won't warp and is much cheaper then cabinet quality plywood. Not sure
why you would use MDO, unless you were building an outdoor cabinet.
Here in Hershey, PA, MDO goes for $75/sheet, compared to $20 for MDF.
Also, if you paint it according to another article FWW did, you can
acheive excellent results.
I have made floor to ceiling cabinets that are 4' wide on which I store
books and have not had a problem with sagging. Some of those are 3-4
years old. When I made those cabinets, I routed dados in both sides,
as well as reinforcing the front with a Face Frame through the midpoint
of the shelf. On the back I used a backer board and some L brackets at
the midpoints, to provide additional support, but also to secure the
cabinet to the walls. MDF will sag over a 5' length if not supported
properly, but most hardwoods also will, just not to the same extent
that MDF will.
You could always make the carcase out of MDF and choose another
material for the shelf. The end result is using MDF will save you