There is a minimal gap between the phone casing and the sim tray cover, so of course grease and crap is going to collect in there. It is a pure white phone, so anything slightly off white is going to contrast the beautiful finish. It's not a big deal IMO.
You can't really go by the store models anyway. People are nasty and don't wash. I like to keep my hands very clean.
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You must have a dodgy phone, because the sim tray on my 5c is completely flush with the casing. If I run my finger down the side, all I feel is the hole for the sim removal tool.