I think it depends, if you use free asset packs I do not think any of these is necessarily more difficult than the other, although if you make your assets from scratch 2D is likely easier. 2D also makes it easier to optimize for performance. But 3D also has its advantages, I find it makes level design a bit more intuitive and animations are much easier (you can basically attach an animation to any existing rig, which allows you to reuse animations, whereas with 2D you need spritesheets for each model)