Plywood. Except specifically for doors, and especially if you haven't built cabinets before, MDF is awful stuff to work with - monstrously heavy, easily damaged, and dusty, dustier, dustiest. Iron-on edge banding works miracles with plywood edges. The perception of quality is more likely to result from other design choices, and from the quality of your paint job, than from the choice of material for the box.
I hope I'm not insulting you by suggesting it, but there's an excellent Best of Fine Woodworking book, called Designing and Building Cabinets, that has some outstanding design and construction ideas, including some specifically for plywood, some specifically so that a cabinet doesn't just look like a plywood box. It's been very useful for me, anyway.
IMO.
- Bob