Costs
Why would you even care about the brand name? The Nvidia 680M is around 20% faster than the AMD's best 7970M, and even the later is over 50% faster than your 6970M. Seems like a no-brainer to me which card to pick.
Ever heard the term 'cost analysis' and bang for your buck - given all the desires the Fanbois have for the latest iMac, it would seem US$10,000 would not cover what is desired.
For your info, and gleaned from several websites focusing on GPU's, as far as bang for buck is concerned, the AMD 7000 series is better than Nvidea.
I actually want a iMac at the present price point, not US$1000 more - just check the figures, DDR3 1600 RAM currently expensive, 1333 RAM currently cheap, existing displays have lowered considerably since last year - the RETINA display alone would be US$1000 ball park - I'm actually surprised people have not put Sandy Bridge Xeon's on their wish list, together with IT off SSD.
It appears to me from where I'm sitting - this being Hong Kong - that many of the Fanbois want to pay huge sums for Apple branded kit, a bit like a sports car being a penis extension - its sad really.
I truly wish you lot would do a cost analysis, add 30% Apple mark-up for rebranding Foxconn goods, and then tell me realistically what we can expect for US$2,500 - indeed, using my Educational Discount I can purchase the present iMac BTO with upgraded i7 CPU for US$2,150, 16G RAM is US$75.00, AppleCare for US$100 and a nice WD FW800 3T external for US$180 - so, for US$2,500 at present price point I get a bloody good machine with APP, maxed out RAM and decent GPU - this is known as COST ANALYSIS - and I'd elect the machine to last me a minimum of two years without having to traps off to Apple Store every six months to have LCD panels replaced - last one this week in a 10 month old machine - I advise Apple to address QC and keep price point acceptable, not try and sell to Fanbois and the US 1%'er's, which many posters seem to belong too!!!!! WE ARE AFTER ALL DISCUSSING A iMAC AND NOT TOP END WORKSTATION WITH US$10,000 MONITOR.