I saw some people complaining in a few other threads/websites that MH3U for the Wii U looks better than MHXX for the switch, so I thought I would compare them myself using the Deserted Island map.
MHXX:
Much better environmental texture quality
Environments themselves have a higher poly count.
Water is of far better texture quality and also has reflections
Much sharper shadows
Anti-aliasing
Dynamic light shafts
Performance more consistent
Moderate frame-pacing issues
MH3U:
More pronounced bloom effect
Occasional performance issues
EDIT: MH3U for the Wii U still uses Bilinear filtering while according to Digital Foundry MHXX for the Switch using anisotropic filtering.