Well man I have seen none of them but having seen the M7 in both black and silver version, I can definitely say go for the silver version not grey because it's closer to black than to silver. Besides, i have seen somewhere in here a poll about the colours (in general also I think) and i was surprised to see around 80% of people choosing metal grey.
I for myself I prefer the silver version which is closer to white and closer to original M7 colour.
Finally, the grey version is more prone to visible scratches since once scratched, it will show original aluminium colour. However, the silver whitey one will have much less visible scratches because it's closer to aluminum colour!
If I will get this phone, and I'm still thinking about it having typed this on my LG G2, I would get the silver version definitely.
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LG G2 D802 - CPU binned 3
ROM: CloudyG3 ROM v1.2 android 4.4.2
Previously owned devices: Nokia N96>HTC HD2>HTC DESIRE HD>SG NEXUS>HTC SENSATION XE>SGSII>SGSIII>LG NEXUS 4>LG G2
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Damn! I think I got it...
I got ORD => Obsessive ROM-updating Disorder => Click here!