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Two options to consider....
Graphic designer as a hobby? If so, I'm sure the i5 with extra RAM will do the trick (for today's needs). Being a hobby, one doesn't mind waiting 10ths of seconds extra.
If graphic designer as a "professional" (especially if running your own company), then get the i7 with more advanced graphic card. For depreciation costs for this tool, buy the highest model i7 iMac under your business name. And if a business tool, get "the business" to buy lots of RAM for your future iMac "tool" as well.
Always size "the tool" for 2-3 years from now - not for today's processing needs. Especially if you see graphic design apps becoming "processor hogs" in the future...
Hope these other considersations help as well...
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