The 2014 model is based on the Haswell architecture and the newer one (current) is based on the Broadwell architecture. Single thread performance between the two is about the same (higher IPC on the Broadwell, offset by the 4MB cache on the i7 vs. 3MB on the i5 and 100Mhz difference in speed). The i7 can do 4 threads in total vs. 2 in the i5 (through Hyperthreading), so there is a bit better multithreading performance.
Biggest difference is in graphics performance, since Broadwell is considerably faster than Haswell at graphics, and helped further by the faster memory in the newer machine, 1866Mhz vs. 1600Mhzs in the 2014 model.
Try also looking at Apple's refurbished stores for cheaper units. Unlike many other brands' refurbished goods, Apple's refurbs are identically new, squeaky clean and not a stain, scratch or hair to be found.