Plastic is cheaper than glass, it rattles in the wind, though you can stop that by assembling with silicon sealant round the edges, and it will stay warmer. If you go plastic check out the light transmission rates, different plastics vary.
Glass is better in terms of light and heat retention, but it is heavier and breaks, I have had a storm take out the end panels of my big greenhouse, and it can be nasty if you trip and fall into it.
For a lot of things a cold frame is a much cheaper and more easily assembled option, for what you want most things would be outside other than in winter. I used to have a cold frame made with insulating blocks of foam used in wall cavities and an old double glazed unit on top, that stayed well warm all winter.
I warm the greenhouse with a heater made from an old jar with a hole punched in the lid and a tube of rolled up tin through it. The wick is made from rolled cotton wool pushed through the tube. It burns paraffin and goes under a large terracotta flower pot, the whole pot gets warm, but as you have probably guessed, I am a cheapskate