If you do a lot of music, then the additional ports in the 2015 model will no doubt be a massive benefit for connecting through to various interfaces etc (without additional cost)
I've just invested in a 2015 model. I have no USB-C devices and I do a lot of photography so use the SD card slot all the time. Also magsafe has saved my MBP a lot of times, I'm stunned they've dropped it.