How do I decide which end mill to pick?

End mill choice depends on the material you are cutting and the intended use case.

For softer or non-ferrous metals, you should choose a 2- or 3-fluted end mill. This is because softer metals can handle higher speeds on the mill and produce larger chips, and end mills with fewer flutes have larger gaps where the chips can go. For harder materials, you should opt for more flutes, as end mills with more flutes are stiffer.

Plan to choose the shortest end mill that will accomplish your intended task to reduce tool deflection and noise. Wide end mills are good choices when you require more rigidity or want to remove material more quickly. Narrow end mills are helpful for producing smaller internal features or fillets (tighter cuts).