The cooler is better and a bit bigger on the Nitro +, but the Pulse is sufficient in most cases (pun intended). The VRM is also a bit different but for air cooling, either would be more than sufficient.Stock frequency is likely also a bit higher on the Nitro+.
Certain vendors bin their chips (MSI had the same PCB for certain Armor and GamingX 10-series models and put the better cooler on the higher clocking chips), but I haven't seen anything that suggests that Sapphire does this. You might still achieve a higher max frequency (both OC and stock) with the Nitro+ because of voltage-frequency and power limit settings on the Nitro+, but the actual differences should be minimal.
These things _can_ also be customized with a BIOS mod, but you really shouldn't do that unless you have Dual BIOS on your card (if your custom BIOS doesn't work, you can switch to the other BIOS, if BIOS fails on a single BIOS card the card is bricked until you can reload a working BIOS). This is the only truly useful advantage of the nitro+ over the Pulse; the dual BIOS capabilities.
But... While Dual BIOS capabilities are cool, most of us will only ever touch the frequency slider or possibly the power limit slider... Which are both available through OC software like MSI Afterburner, or Radeon settings. Radeon settings even have voltage-frequency settings available, with no BIOS modding required.
TLDR; Either are fine if your airflow is fine and you don't care about BIOS modding and absolute max OC, Pulse is fine. It's a question of // Æ S T H E T I C S // , and if your case/room/country is hot, cooling capabilites.