In my experience keys and change don’t matter. The metal is too soft to scratch phone screens. (Besides being soft, keys and change also tends to not have much in the way of really sharp edges. Maybe new keys do, but don’t they get quite smooth over time? The aluminium case is potentially a different story.)
What matters is sand. (For what it’s worth my iPhone has three, four scratches after a year. I’m pretty certain they are all from when I slid my phone around on some surface with a grain of sand between surface and phone – which I usually try to avoid, but oh well. That’s it.)
To be honest, though, for me personally scratches don’t matter. In normal usage they are invisible and while they do annoy me I would rather the phone be more resistant to falling down.
What matters is sand. (For what it’s worth my iPhone has three, four scratches after a year. I’m pretty certain they are all from when I slid my phone around on some surface with a grain of sand between surface and phone – which I usually try to avoid, but oh well. That’s it.)