Generally 2 & 3 flute is what you use for aluminum & soft materials. 4 flute for steels. Also use 2 flute for slotting not matter the material.
The ones listed for aluminum that are actually designed for aluminum (some people on ebay don't list accurate descriptions) have a high helix (usually like 35° & up, please correct me if I'm mistaken) & they work awesome. Usually you'll find these in 3 flute but they are available in 2 flute also.
2 flute was always normally recommended for aluminum but I now use 3 flutes like many others for aluminum when I can. Much better surface finish. There's a video by Stefan Gotteswinter where he shows that he uses regular 3 flute endmills for roughing steel.
Everything posted above is for endmills in general which applies to roughing & non-roughing. For roughing endmills you'll find 2 general different types, fine & coarse... profile, pitch, teeth, not sure of the actual terminology. Fine is better for hard materials, coarse is better for softer materials (mild steels included) & faster material removal. Then there are specialty profile roughers, like Niagara makes some interesting ones.