High speed steel tools have a pretty good tool life in aluminum, but of course carbide is better. Carbide will hold accurate size for a longer period.
There is a special high helix flute design for milling aluminum, and this works much better than a standard helix. It is easier to find this high helix flute in HSS, than carbide.
In steel, you pretty well need flood coolant to use HSS cutters, whereas carbide often doesn't stand up well to the rapid heating and cooling cycles, should coolant flow be impeded temporarily for any reason. Carbide is often used dry, with an air blast (or lubricated air blast). It is important to not recut the chips, and flood coolant will often not move the chips out of the cut zone as good as an air blast will. By air blast, I don't mean a wide open 1/4 inch air hose, a small nozzle will do if properly directed.
It is pretty important to have the proper feed rate available for carbide, too. Beginners tend to overspeed and underfeed their cutters. Better to take cuts that are not all that deep, but at a high feed rate with carbide, depending on what your machine can handle.
That's my 2 cents
