Which interface is better for my OLED display?
They are all comparable, with the tradeoffs being Pins vs Speed. It really boils down to your preference. How many free pins do you have (Can you spare 8 data + 4/5 control pins for parallel data?), are you already using SPI or I2C? The I2C version will limit you to only 2 lcds without using multiple I2C buses or buffers/switches, but only requires 3 data lines (SDA/SCL/Reset) and simplifies the command/data code by including it into a byte instead of requiring external pin toggling.

If you were me, I'd use i2c, but mostly because I prefer it. If you are already using SPI, that be the way to go.

As for 6800 vs 8080 lcd addressing, they are pretty similar, and have no performance difference. The 6800 interface is a bit easier to implement, and for the most part, is exactly the same as the common HD44780 character lcd interface.