Honestly, you should go check them out in person.
Otherwise compare needs vs differences:
Lumia costs less vs M8
Snapdragon 800 vs 801. Therefore Lumia is slightly worse in CPU, GPU, RAM, and battery life performance. The GPU sees ~30% improvement, but considering the S800 GPU isn't tapped yet, that doesn't really matter.
Lumia back camera is better
M8 has sd card support
Plastic vs Aluminum
Stereo external speakers vs mono