I was just thinking about this. I also have a 2012 MBP (2.7/16/756) and was thinking about upgrading.
From what I read, the main difference is the SSD, it is a PCIe based flash storage in the 2013/14 vs SATA in the 2012. There is also the processor, graphics card bump and a faster wireless 802.11ac. There may be more, but from what I read these were the main differences.
I decided not to do it because it's faster, but only marginally faster. I likened it to a Porsche Turbo and a Porsche GT3. They're both pretty fast and you likely won't be able to tell the difference or drive it fast enough to test it.