On a hot summer's night, not much difference...
True - Happoshu (imitation beer, or, as I call it, "near beer") is a low-malt mixture and sells for less than regular beer. I feel happoshu tends to have a sharper taste.
However, if you are looking for a crisp beer taste and are on a budget, then nobody will think the less of you for trying a happoshu. The sit along with the so-called "real beers" in the liquor aisle of any store. Properly chilled, they are right up there with being a great, refreshing way to enjoy a summer night.