Arduboy: 128 x 64 screen
Gamebuino: 84 x 48 screen
so it is somewhat bigger, but then again, you'd only be albe to play one game on the arduboy if you take it with you, due to the lacking of the SD-Card.
Oh, and another dis-advantage of the arduboy is that it doesn't have a good speaker and the battery life is shorter. Oh, and the gamebuino looks cooler
And the gamebuino also has one more button
The I2C and ICSP ports on the gamebuino also allow for custom hardware modules