Stainless Steel is more scratch resistant than anodized aluminum and if you do manage to put a scratch on Stainless it is really hard to see.
If you're dead set on Aluminum, Space Grey is probably the worst color to get because scratch the surface on a Space Grey case then the Aluminum underneath shines bright reminding you "Damn!!! Should have got Stainless Steel". Remember ScratchGate with the Space Grey iPhone 5.