Hi,
I had asked a question about Adafruit displays a few months back. It was centered around LEDs and so I am not sure if OLEDs are supported. It might be worth a look. Here is the thread:
On your other question, I just opened a thread on SPI OLEDs. In my case, I have some SPI OLEDs which I purchased from eBay and work fine on Arduino, but I am looking for a Photon library. The Adafruit units may very well have better support then my cheapies. 
From an I2C vs SPI angle, my screens are SPI and while they are fine, they require more pins than an equivalent I2C setup and so I would purchase I2C units another time.