Assuming these are the correct cards I'm looking at:
Sapphire nitro
Gigabyte
The gigabyte one has 1905mhz boost clock while the Nitro has 2020mhz
Sapphire nitro also has some built in RGB if you like that sort of thing. It's up to you if an extra 115mhz and some RGB is worth $30 extra
It also might be worth noting the gigabyte one has 3 display port and 1 hdmi output while the sapphire nitro has 2 hdmi and 2 display ports. That could be worth keeping in mind depending on what monitor(s) you're using but even then, adapter cables are only like $10 on amazon
And another question: how big would be the performance difference between the OC card and the Nitro+ ? Thanks.
The 115mhz difference is a rather small performance difference, not worth paying an extra $30 if you ask me, unless money isn't a problem.
Make sure to also keep in mind the size of your computer case you're getting. Can't install a larger video card if your case doesn't have the room for it.