There may also be some ramifications for adding Apple Care as well; you can do it for sure on the new one, but I'm not sure how Apple Care works when you buy a pre-owned machine from someone who does not have the extended Apple Care on it.
You will see a major improvement in storage speed regardless of whether it's a 2014 or 2015. It's kinda like, just how fast does it need to be before you can't notice the difference anymore in normal tasks. I would have seriously considered getting a new 2014 over a new 2015 if the price was right; however, I was able to get a new 2015 for $100 more than a new 2014, so to me the ForceTouch trackpad, faster SSD and an AMD GPU made it worth the $100. Regardless of the specific speeds of the choices in GPU, I am of the opinion that now that Apple has AMD across the board for all of their computers with discrete graphics, this can eventually become a "life cycle" determinant. As they roll out updates to OS X, the configuration of your hardware becomes the reason that you can or can not take advantage of certain updates. Now that AMD is ubiquitous, I would not be surprised that some time down the road all pre-AMD systems become ineligible for updating OR at the very least, won't be able to take advantage of new features. That may not be a big deal to you if you update your hardware frequently, but I just upgraded from a 2008 24" iMac to this MBP, and ONLY because it became too expensive to repair after a recent MOBO failure. I hope to get similar life from this, so I opted to get one as up-to-date as possible. I would have waited for Skylake, but time was not on my side.
