We have a 35 year old plastic snow shovel which works well on the rare occasions it is used. As you say it does need a metal edge as ours is wearing out. But I am suspicious of the quality of modern plastic because of the practice of adding reclaimed material to the melt. A 10% addition results in a 50% reduction in the material properties, so you could end up with a brittle product which readily disintegrates.
I'd go for the aluminium.