If you will be ignoring the addition of 3D I would say that is not a logical upgrade path in relation to the investment.
Obviously the investment being worthwhile will vary per individual but in my honest opinion the 5020 is not $1800 better than the 8700 regarding over-all 2D image quality.
With that said the 5020 is an upgrade from the 8700 (just how much of an upgrade is subjective).
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