For blenders, is Stainless Steel sturdier than Glass? My grandparents intend to blend warm (≤ 70°C = 158°F) soups in addition to fruits and vegetables. To them, glass feels more fragile and untrustworthy compared to stainless steel. So is stainless steel stouter?

They know that opacity is one disadvantage of Stainless Steel; they haven't lighted on a transparent stainless steel container/jar.
If they are going to blend hot soups, go with stainless. Glass can (and does!) crack due to the thermal issues. If they are religious about cooling their soups down to room temperature glass can be ok. But I've definitely had glass jars crack when blending hot soups. Tempered glass does not solve all problems.