Carbide will stay sharp longer, but is more expensive up-front and over time. HSS will dull faster but also takes a marginally sharper edge (but if youll notice the extra sharpness is a different conversation), and is cheaper both up-front and over time. A carbide cutterhead adds about $2-300 to the price of a machine, last i checked the prices, and replacement inserts run anywhere from $5-$15, again when last i checked prices. Machines with HSS knives are cheaper, and the blades themselves can be resharpened several times as well.
One bit thing to note, carbide is a lot less forgiving than HSS is, as far as cutters go. Carbide is hard but very brittle, so doing something like clipping a staple will usually shatter at least the edge on that insert. Now, you dont want to hit metal with HSS knives either, but at least then you might get away with a nick that can be sharpened out, instead of shrapnel