is it wise to go back for the d90 ?

well my sister in the usa and will come to egypt soon and she asked me to get me something from there and i thought well why not upgrading my d40x ? and first i thought of the d5100 then i found the d90 almost the same price but i don't know is it old enough to make me go for the d5100 ? i know the d5100 has better iso and quality overall but i don't care for the difference cause i'm sure both are good and both are good upgrade for my d40x .. so what do you think ? should i get the d90 ?
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