adobe media enconder export questions

Element was designed to leverage your GPU, it uses a lot of the "tricks" games use to render images, so in theory, using your GPU will help here BUT... GPUs are optimised for game playing, as a result they can get around issues with skipped frames etc..., AE requires each frame to be rendered perfectly, so if you can get away with rendering on a GPU, go for it, but if your render fails, software only takes longer but gets you there.  

Software Only is actually a troubleshooting option for when you have GPU issues; this setting bypasses the GPU and is slower than using CUDA/OpenCL/METAL.

Unless you suspect you're having GPU issues, leave this set to CUDA on your system. Your CPU and RAM are actually more important to the rendering process than the GPU.