In fact, the early 2014 revision was the minor bump of mid 2013 since all it got was +100 MHz on the Core i5 model and no change at all on the Core i7 model.
The early 2015 model has way more changes by comparison:
- Broadwell processor vs Haswell
- HD 6000 graphics vs HD 5000
- Thunderbolt 2 vs Thunderbolt 1
- 4x PCIe 3.0 SSD on the 13" vs 2x PCIe 2.0 previously (max speed 2GB/s instead of 800MB/s)