Generally speaking, glass is better. Glass isn't absorbent and won't emit chemicals.  It will remain crystal clear for viewing and keep liquid fresh. It depends upon the thickness and quality of the glass, however. Glass bottle feeders featuring an inverted antique bottle embossed with writing and lettering will be thicker and heavier than some blown glass artistic styles. Having the glass in hand to determine the durability is important if you think your feeder will be exposed to high winds or a fall.  Plastic tends to develop a "cloudy" coloring over time, especially in the heat, and the softer plastics may have toxic chemicals that will leach into the liquid. Plastic is harder to clean because of these factors. You will want to ensure the plastic is shatter-resistant, has a heavy material to sustain drops, and banging against poles and trees.