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I think any future buyer would not care or select your system over the other used systems out there because of the 560 vs 555. A 512 GB of storage versus a 256 GB is a differentiator, but not model of the graphics card.
 
I'm also having the same dilemma and I don't play any games or (currently) do any video editing. I do however, use Lightroom quite a lot, so will the GPU make any difference for that?

For reference I will be upgrading my 2017 base-nTB MBP 13", primarily because it lags a lot in Lightroom, and I also have performance issues with InDesign and Illustrator.
 
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