From a technical standpoint, using pixel art takes way less space and may load easier and faster. ......
in many cases it will be opposite.
For the same character - a rig, mesh and texture and maps for a 3D character can take up quite minimal space compared
- to the same same animated 2D version, especially if using a say 100 reasonably high res frames.
but back to subject
if you want dynamic and exciting animation then pixel art and skilled animators still rule.
3D models mostly always look stale and rigid, it is so much harder to make them come to life than 2D animation, and the people with the skills needed to achieve this are still few and tools involved are just maturing in this respect.
However one notable exception is Blizzards Overwatch. It has really turned the tables in this respect.
They have really pushed the boat out in terms of applying 2D animation processes (such as stretching/squashing/tweening/easing/exaggeration/anticipation etc) into the 3D realm, in game, and the results are astounding.