I'll just share a few of my views here, but don't take anything I say as the final word on the subject, about the time I think I know something I find an exception, but here goes.
Four (or more) flute mills are stiffer and stronger than two flute mills and can generally cut at twice the feed rate, so for maximum material removal and minimal side deflection they work best. They also wear slower, you're cutting the same material with twice the teeth so each tooth wears less.
Two flute mills cut slots that are more accurately located. With a two flute mill, at one point the two teeth are cutting on each side of the slot, so there is no side thrust to cause the cutter to deflect sideways; with a four flute mill when two teeth are opposite each other across the width of the slot another tooth is cutting sideways into the thickest part of the cut and causing the mill to deflect sideways. Two flute mills are much better for most cutting of aluminum, especially if you don't use flood coolant to flush out the chips. The extra space between the flutes, especially in the smaller sizes, makes it easier for chips to clear out. One trick I use on the manual mill is to use compressed air instead of coolant to blow the chips clear when machining aluminum.
End mills come in center cutting and non-center cutting styles. I don't buy anything except center cutting mills if I can. Center cutting types allow you to "plunge" (drill) down into the piece as well as cut sideways. Even so, do not crowd an endmill down into the workpiece like you would a drill, the clearance angles on the end of the mill are not the same as a drill.
There are all sorts of materials used to make end mills - High Speed Steel (HSS), carbide, coated carbide, cobalt alloys (nonferrous material content), and ceramics. For harder steels carbide is probably best, but for mild steels, aluminum and brass just about any material works fine, especially for the hobby machinist. A lot of the development in ceramics and coatings is meant for high speed production machining and doesn't give too many advantages to the hobbyist.
If you machine a lot of aluminum having a few high-helix multiflute end mills around is handy, they tend to clear chips out nicely, but you can get along without them.
If you plan to machine soft material like copper do not rely on a friction collet to hold any end mill with a helix. Use a Weldon (setscrew) shank type so the end mill can't screw itself down into the workpiece.
Hope this helps. I suggest buying a a couple of each type and watch what happens when you use them.