They're actually both the same active ingredient: Super Absorbent Polymer gel. Maybe not the same formulation though as there are a few different chemical formulas for it. Generally speaking SAP is two way, it's just not as quick to absorb moisture from the air as it is to release it. Also, depending how it's made, the RH it maintains can vary from one type / manufacturer to another.
Additionally, the membrane on the water pillow interferes with the absorbency of the SAP in the pouch. It also can quickly turn into a mold farm.
They actually do have a lifespan, but it's unlikely you'll hit it using them in a humidor. Dump them into your potting soil with your plants and they'll degrade in 5-7 years.
Thunder.Gerbil wrote: