Two flute gives more chip clearance for aluminum. It doesn't matter thaaat much. I usually just grab whatever is on hand. You'll find machinists that'll tell you that it matters and machinists that tell you that it doesn't.
And yes, if you run a 2-flute end mill on steel at 600 rpm then you'll have to go slower to get a real nice surface finish, but on aluminum running a 2-flute at 3k-6k it just doesn't matter that much.