Depends on your use case. You go from quad-core to dual-core and sacrifice screen space but gain portability. You get a much faster SSD as well. I have the 2015 13 with 16/512 and the highest processor. It can struggle with a heavy office workload. You're taking more risk with laptops getting older and older too. One day, they can just die and if they're out of service, you can be out of luck.
I mainly use a 2014 MBP 15 and a 2015 MBP 15 and I love these two machines. I rarely use the 2015 MBP 13. I'd love a 16 but I can't justify one as my existing hardware is fine.