Just curious to know what input people are using and/or which one is better if there is one, or if it's just preference.

Most people are using controller for most of the game and will pause at certain points to switch to K&M. The biggest example is for merchants since you can scroll through the menus very quickly with the mouse wheel but it's also used for the advantages of faster and steadier aiming at many points in the run. I would recommend watching Frenchie and Yuushi to see where they use K&M. Using it for most of the things they use it for is highly recommended. Besides for that, it's theoretically better to just use K&M only for the whole run but that's only realistic if you're someone who is very accustomed to playing with it already. That was the case for MikeWave and he's the only person to ever effectively use K&M only for speedruns