The 900 GTX and 900m GTX Maxwell series are suppose to be really close to each other in processing power. Before that the m ones would run a lot slower then the full card versions. For that matter there wasn't even an 800 series for the desktop cards. The reason you can't really compare them is because you can't put the 980m and the 980 in the same computer for testing. And since you can't replace the mobile ones just buy the highest cost one you can afford that has all the other feature you want and the fastest video card available.