As for the SSD R/W speeds they are 1500 MB/s on the 2015 MBP and 3000/2500 MB/s on the 2017 MBP, right? So the 2017 would be 24/20Gbit/s, which is close to Thunderbolt 2 on the 2015 MBP. If you don't buy enough internal SSD for your video editing needs you would want a fast external SSD which would perform similarly on both MBP's.
With that I mean a Thunderbolt 2 SSD on the MBP 2015 would have 20Gbps write, which is the same as the internal 2017 SSD. Not sure if those are cheap or if there are faster ones. But future MBP's will perhaps have updated internal SSD speed equivalent or closer to TB3 max speed. Internally, the 2017 is faster. If 256 GB's enough.
Otherwise it's either pay more for 2017 internal SSD or pay more for external TB2 SSD for the 2015 MBP.
Edit: Just looked, a QNAP TX-500P with 5x Samsung EVO 850 250GB is about $1100, about the same as the price difference between the 2015MBP with 256GB SSD and the 2017MBP with 1TB SSD.
As for the screens, there will surely be further updates that will make the 2017 seem old. HDR, OLED, true 10bit etc
Edit: The 2017MBP probably has a brighter and nicer screen, but keep in mind icons and wallpapers could be tweaked for the wider color gamut, if you don't utilize it for DCI-P3 video editing, then nothing takes advantage of it. Perhaps your own photos shot with iPhone 7/8/X.