From what I understand, aluminum is endlessly recyclable. Plastic can only be recycled a few times, and I don't believe it can ever be recycled into food grade plastic again. Also, the plastic label cannot be recycled. Shipping-wise, plastic is lighter, but aluminum uses more available space. They are both round, but plastic bottles are wasting space at the top where they dome in, as well as the space taken up by the stackable plastic shipping pieces. I don't know which allows for more fuel efficiency: less empty space resulting in more liquid shipped per truckload, or lighter containers resulting in less fuel needed for propulsion. I can't think of many ways to reuse aluminum cans, and can only think of a few for the plastic bottles. Any uses for either will last a long time, which means you won't be doing much in the reuse section of the 3 R's.

As long as they aren't kept in direct sunlight, I don't think it should be a problem in your zone. I've used the baggie method more often for tenting and the only times I've roasted ones due to heat was when I forgot about them somewhere where direct sunlight got on them for too long.

I've planted my cucumbers over a week ago. They were wintersowed and growing already outside. I covered them with bottles, same as you describe. I've mulched around with shredded leaves pretty high. So far, so good and they survived a few quite cold nights. I know, it's crazy early but everything sprouted so early. I want to do this with a couple of tomatoes and sunflowers. I have to wait till weekend, no time right now.

it's a very crude version of a drip stem. The bigger the holes the less efficient it will be. Eventually, the small holes will plug up.