Always buy the latest mac, because they bring newer versions out so often, the older models devalue quite quickly.
What I do is buy the latest iMac, sell it before 12 months is up and buy the latest model. That way it costs me around £4-600 per year to have the latest top spec model.
After 12 months the warranty expires and you need to purchase Apple Care before this time. Having a mac without warranty or Apple Care seriously devalues it.
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