They describe different things, which is why the comparison confuses people. Dagger describes a symmetrical blade sharpened on both edges for its full length. Hollow describes a concave primary grind ground on a wheel. A blade can be both, either, or neither, because one is about symmetry and the other is about cross-section.
A dagger profile is symmetrical about its centerline and sharpened on both edges along their full length. That is a description of the blade's overall shape and how many of its edges are sharp.
It is not the same as a spear point, which is symmetrical but sharpened on one edge only, sometimes with an unsharpened false edge on the other.
A hollow grind is concave: the blade dishes inward between spine and edge because it was ground against the curve of a wheel. That leaves very little steel behind the apex, which makes it keen and easy to sharpen.
Hollow is a grind geometry, sitting alongside flat, sabre and convex.
Because reading them as alternatives leads to the wrong purchase. If you want a blade that cuts efficiently, you are asking about grind. If you are asking about a double-edged profile, you are asking about shape, and that is a category we largely do not stock.
Our range is single-edged working knives, tools and collectibles.