First off, you left out the most important ingreedient, like what are you cutting with them!
HSS is a general purpose cutting material, good for mild steel and non-ferrous metals.
COBOLT is the next tougher grade of cutting material, it is very strong and holds an edge when cutting harder and abrasive materials.
Carbide is the best for high feeds, high speeds, deep cuts, and tough materials, BUT
There are sevral grades of carbide, C2, C3, C4, C5, C6....
C2 is the softest
C6 is the strongest (known as lifetime edge) and used in impact drills for masonary, and for cutting treated titanium.
So match your materials to the cutters, then scrounge up the best price!
For more info on Carbide grades: Click Here