A colleague of mine bought a pair of H&M heels (leather) last year and by the end of the day her feet were almost bulging out of the shoes. They were very uncomfortable on her despite the fact that she drove to work, then sat in the office most of the day (i.e. not much walking). I think Zara's shoes (or anything else for that matter), while still low-quality) are better made than H&Ms. Maybe it's because Zara actually produces an entire shoe collection each season and has somewhat mastered this craft, while H&M only produces a few boots or heels here and there.
What irks me most about both brands: even if the upper is leather - the lining is usually still plastic, which isn't the best for feet. My colleague complained about her feet sweating more than usual in those shoes, which isn't surprising given the poor quality of the insole. I personally think a plastic insole is just as bad as a plastic upper.