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While the CPU is pretty much identical, they use different GPUs (in case you buy the model with dedicated graphics). Also, the 2015 one comes with bigger battery and force touch trackpad which is totally worth it
 
Get the 2015. Better touchpad, GPU, and the displays are most likely still covered under the stain gate repair program AND there’s a chance you can get a free battery replacement (if the serial is within the range affected) due to the battery recall
 
From 2014 to 2015 The processor was upgraded and more efficient for better battery life, the GPU was faster, and I believe that was the first version they introduced with the force touch trackpad.
 
I am wary of any retina MacBook Pro with an aftermarket SSD upgrade. There is long thread here that goes through the issues, but the bottom line is there have been various issues with adapters and 3rd party drives, and sometimes loss of performance when compared to the original Apple OEMed SSDs.

I also am concerned that Apple might use this upgrade to refuse to service the machine on other extended warranty such as screen de-lamination.
 
I also am concerned that Apple might use this upgrade to refuse to service the machine on other extended warranty such as screen de-lamination.

I'm pretty sure that this is illegal in the US. Can't invalidate the warranty on the screen just because the SSD has been replaced unless you can specifically tie it to the failure. If you read through their warranty policy they talk about this in several places; for example they may refuse to fix "an original product that is inoperable due to unauthorized modifications."

A common screen defect should be clearly unrelated to a straightforward SSD upgrade.
 
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