According to Canon, the 1D2 had a heavier AA filter. For this reason I would buy the 2N. The 2N also uses picture styles which isn't important to many however it will give more functionality with DPP.
This is straight from Canon.
"• EOS-1Ds Mark II: the need for sharpening and recommendations are very similar (subject, of course, to a photographer’s
individual opinions) to the Mark III series cameras.
• EOS-1D Mark II N: again, the amounts and type of sharpening required are very similar to the newer EOS-1D Mark III.
While critical users may discern very slight differences, and prefer to modify sharpening in one vs. the other, their essential
character is very similar
• EOS-1D Mark II: this one is definitely different. It required a much greater amount of sharpening, either in-camera or
at the computer (some users got their best results applying both). Unsharp Mask settings of 300% or more were often needed
in Photoshop to get real “bite” out of files from the 1D Mark II. Again, to be clear, this is only the EOS-1D Mark II — the
revised and updated EOS-1D Mark II N model tended to need less sharpening, even if none was applied in-camera."
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