Reuniclus is bulkier.
Here's a comparison of how well either can take from a Banded Krookodile (Why this Pokemon? For argument's sake.):
>252 Atk Choice Band Krookodile Knock Off (97.5 BP) vs. 252 HP / 252+ Def Eviolite Duosion: 320-378 (95.8 - 113.1%) -- 75% chance to OHKO
>252 Atk Choice Band Krookodile Knock Off (97.5 BP) vs. 252 HP / 252+ Def Reuniclus: 380-450 (89.6 - 106.1%) -- 37.5% chance to OHKO
Reuniclus and Duosion have the same EVs, level and items. But Reuniclus only has a 37% chance of fainting to that hit, while Duosion has a 75% chance. Reuniclus all the way.
Hope I helped!