Edit: My thoughts are regarding the dGPU models as you mentioned that the price doesn't increase by much when adding 512GB to the 15". I also saw discourse in regards to the 750m vs R9 370x, I must have glossed over your second post. If you are considering the base models, only the 2014 and 2015 are available in the refurb store so jump down to the last paragraph and ignore the bits about the dGPU.
--------------
If you are considering the Mid 2014 15.4” MacBook Pro for the purpose of savings than you should really take a look at the Late 2013 models in the Refurb store which offer even further savings. They are the same computer. With that said the Mid 2015 has key upgrades over the 2014 you should consider.
Mid 2015 > Mid 2014
Mid 2014 = Late 2013
Late 2013 > Early 2013
The Late 2013 was a substantial upgrade to the 15.4” MacBook Pro. The processors were upgraded to “Haswell” architecture, The graphics cards were upgraded to the more powerful NVIDIA GeForce GT 750M graphics processor with dedicated GDDR5 memory alongside the "integrated" Iris Pro 5200 graphics processor. "Early 2013" models have 3-stream 802.11a/b/g/n Wi-Fi and two "Thunderbolt" ports whereas the "Late 2013" models have faster 802.11ac Wi-Fi and two faster "Thunderbolt 2" ports. Storage was upgraded from SATA 6Gb/s to the significantly faster PCIe interface.
The Late 2013 and Mid 2014 are the same computer, both using the 11,3 identifier. Both using the Intel quad core “Haswell” processors and the NVIDIA GeForce GT 750M graphics processor with dedicated GDDR5 memory alongside the "integrated" Iris Pro 5200 graphics processor. Other than the differing clockspeed, they are one and the same computer.
The differences between the Mid 2014 and the Mid 2015 are more subtle than those between the Early 2013 and Late 2013 but there were key upgrades made. The Intel quad core “Haswell” processors remained. The graphics card was upgraded to the AMD Radeon R9 M370X with 2 GB of dedicated GDDR5 SDRAM which is notably faster in OpenCL hardware accelerated tasks. The newer models can support a larger external display up to 5120x2160 at 60 Hz via Thunderbolt 2. A notable difference is that across all capacities the SSD was upgraded from PCIe 2.0 x2 to PCIe 2.0 x4, delivering literally double the throughput. The "Mid-2014" models (as well as earlier ones) have a "no button" trackpad with "inertial scrolling" support, whereas the "Mid-2015" models have a more advanced "Force Touch" trackpad. The battery was increased from 95W Hr to 99.5 W Hr, with Apple claiming an additional hour of battery life.