Both are good computers. I think you will like either.
One current plus with the 2015 is that you can find many Thunderbolt 2 docks and enclosures for dirt cheap - my AKiTiO Thunder2 Dock has gone as low as $150, and I've seen TB2 PCIe boxes on sale for under $175. However, USB-C is awesome, and Thunderbolt 3 opens up new possibilities that even Thunderbolt 2 does not. Further & more significantly, the new MBP supports USB 3.1 gen 2, 10 Gbps, where as the 2015 only supports 3.1 gen 1, 5 Gbps. During the life of this system, this will probably become a major plus as many high-resolution displays embrace USB-C & the second gen 3.1 spec, and many external SSD and RAID enclosures will be able to take advantage of the greater bandwidth of gen 2 (that would have historically required TB1/2 [i.e., USB 3.1 gen 2 means super fast external drives without the price premium of involving Thunderbolt].)
- Do you prefer the touch bar or physical function keys?
- Do you have a keyboard preference?
- Do you like the larger 2017 or smaller 2015 trackpad?
- Are better speakers important to you?
- Do you often use the computer in bright rooms?
- Do you currently use many legacy devices?
- Do you see yourself buying many new devices over the next 3-4 years?
- Do you see yourself using one or more 4k or 5k external display(s) during the life of the computer?
- Do you see yourself as getting into serious video editing?
- Is the idea of being able to use one cable to provide power and data to as many accessories as your heart desires something that would be convenient for how you use the system?