both cards exchange blows with the 4850 X2 taking the lead. The 4850 X2 doesn't exhaust since it has 4 DVI ports and generally performs under 2 physical 4850s.
285 GTX gives you less headaches since it's not driver dependant, but the 4850 X2 is the best bang for the buck in most cases.
I personally wouldn't even consider a dual GPU unless it had a noticeable difference in price and or performance.
But thats my 2 cents:D
I agree. If they are about the same price I would stick to single gpu solution.