These two are frequently treated as substitutes and they are really complements.
Metabolic behaviour
Erythritol is absorbed in the small intestine and excreted unchanged in urine, which is why it carries a digestive tolerance above 0.8 g/kg body weight and draws no laxative warning at normal intake. Allulose is also largely excreted unchanged but a fraction reaches the large intestine, and in sensitive individuals intakes above roughly 0.4 g/kg can produce transient GI discomfort. That ceiling matters when you are formulating a product someone might eat in a single sitting.
Functional behaviour
This is where the two diverge completely. Allulose is a reducing sugar: it participates in Maillard browning, depresses freezing point, and delivers a syrupy viscosity close to sucrose. Erythritol is none of those things. It contributes a cool sensation, crisp crystallisation and a clean finish, which is exactly what you want in a confectionery coating but not in a cookie.
Regulatory and cost
Erythritol is approved essentially everywhere and is among the cheapest bulk options on a per-kilogram basis. Allulose is approved in the US, Japan, Korea, Singapore, Mexico and several other markets and is exempt from Added Sugars on the US label, but remains under novel food assessment in the EU. It is the more expensive of the two.
The practical answer
For most applications use both. Allulose supplies browning, freezing and mouthfeel; erythritol supplies economical bulk, digestive headroom and structure; a small fraction of monk fruit lifts the blend to full sugar sweetness. That three-component system outperforms any single ingredient and is the format we ship most often as a co-blend.