Aside from the obvious which is the colour, there are a couple more I would like to add.
The i15 has a dead impact sound. The G15 has a more acceptable tinny tone.
The G15 is hotter off the face at impact. i15 doesn't have too much oomph. Almost mushy-like. Sweetspot also feels smaller than the G15.
I know some will argue that both feel n sound r related but I guess w a driver, the avg consumer will take sound more into consideration as they relate it to distance which isn't necessarily accurate.
Stay Thirsty My Friends...