I like the copper slag better, but I'm kinda partial because that's how aircraft cylinders look when they're new and unpainted.
The walnut shell looks like you ran it under a wire wheel and gave it that grainy "aluminum that's been wire wheeled too much" look.
If you go with bare aluminum, the copper slag will probably maintain its finish longer than a polished finish would.
Although, it would look nice if you accented some of the rough aluminum with mirror polished parts, like the PV window, water pump, flywheel cover, etc.