Which category of plastic, photodegradable or biodegradable, is better for the environment?
Photodegradable and biodegradable plastics are each better for the environment than non-degradable plastics. However, only biodegradable plastic is able to be returned to the natural carbon cycle in a reasonable amount of time, thereby rendering it superior to photodegradable plastic.
Photodegradable plastics are those which have chemicals added that make them brittle under certain conditions, usually exposure to sunlight. Although the plastic appears to be breaking down into smaller more manageable pieces, they are still pieces of plastic and not usable by organic organisms. Technically, the pieces may eventually be broken down into small enough particles for use, but the timeframe isn't established and cannot be considered reasonable yet, although science is still working on improving the products.
Biodegradable plastics break down and can be ingested by organisms in the metabolic cycle. This returns the carbon in the plastic to the carbon cycle in nature. The ability to be re-used via natural cycle makes biodegradable plastics better for the environment, though still not as good compostable plastics, which are biodegradable plastics with a very specific degradation period. The timeframe it takes biodegradable plastics to break down depends on the type and how close to optimal the conditions the plastic experiences are.