It depends on what it is your buying. Both stores are can be tricky, one thing you pull out may be five dollars when the things right next to it may raise to thirty dollars. I got a thirty dollar dress at H&M and almost bought a fifty dollar one at Forever 21 thinking it would only be between fifteen and thirty dollars.
Foreverr 21 is definitely cheaper but so are their clothes. H&M has amazing sales sometimes and their clothe last longer. I also feel like the stick to high fashion details. There colors are according to season also.
Forever 21 is cheaper in terms of quality. The prices are nearly the same, with H&M being a tad bit higher. But the little extra is worth it! You get more life out of your clothes so it would be a cheaper unit price overall.
Forever 21 has a wider selection of "cheaper" items. H&M often has high-quality items that, when you're lucky, you can find at the price you'd see at Forever 21. However, the quality of H&M clothing is far superior to Forever 21.