High-speed steel (HSS) end mills are tougher and cheaper, making them suitable for low-speed manual and general machining of softer materials.
Solid carbide end mills are harder, retain their edge at much higher temperatures, and allow 2-3× higher cutting speeds and feeds — the default choice for CNC, hardened steels, and high-precision work.
If your machine is rigid, you run CNC, and you need repeatability and tool life, solid carbide with the right coating pays for itself. For occasional low-speed jobs and delicate geometries, HSS remains economical.