No case, if you are careful and if you do not care about some usage-scratches.
YES Case, if you care too much about your mobile phone, use a screen protector, a back protector, a big ass bulky case with all plugs to keep your phone from being used at all.
No case, you benifit from the design and the feeling.
With a case you use the phone, in a different way which it isn't ment to be, altough the resale value might be more after 2 years than without a case but you shouldn't care about that. But if you are sure you will drop your phone regularly I would go for a case.