The biggest differences between the iPhone 6 and the iPhone 6 Plus are their size and battery life. The iPhone 6 Plus display is an enormous 5.5" diagonally, while the iPhone 6 is 4.7". Both are larger than the 4" iPhone 5s display.
The larger display allows the iPhone 6 Plus to fit in more pixels (1920 x 1080) compared with the iPhone 6 (1334 x 750). It also allowed Apple to fit a larger battery in the iPhone 6 Plus, which lasts longer — video playback of 14 hours and audio playback of 80 hours, compared with 11 hours and 50 hours respectively for the iPhone 6.
The phones have the same operating system, processor and chip, camera and video recording capabilities. Both phones support Apple Pay. Both are available in 16GB, 32GB and 128GB models. The iPhone 6 is $100 cheaper than the iPhone 6.