Plywood or block board: a decision about span, not quality
Both are good panels. Which one you need depends almost entirely on how far it has to reach unsupported.
Plywood is cross-laminated veneers. Block board is a core of softwood battens laid edge to edge, faced with veneer on both sides. Neither is better in the abstract.
The deciding factor is span. A block board core is lighter and stiffer for its weight, so it sags less over a long unsupported run. For a tall wardrobe shutter or a 1200 mm shelf, that is the whole argument.
Plywood wins where the panel is structural or heavily loaded, where it will be screwed into repeatedly, or where it needs to hold a fixing in the edge. It is also the better choice in wet locations, where a batten core has more to lose.
For a full wardrobe: plywood for the carcass, block board for the tall shutters. That combination costs no more than choosing one for everything and performs better than either alone.
