Since the new 1/4" collet came in for the Pocket NC, I can now use more significant tooling. I don't have any endmills for cutting aluminum specifically, so I'm in the process of picking up some new ones. I've been talking to Wayne (the local CNC shop owner) and asking him about the different aspects of the bits I could get. The one thing he keeps stressing to me is rigidity, which I haven't heard other machinists talk about much, but it makes a lot of sense.
His premise is that you only want to get enough cutting length for what you're going to use. In this case, I will only ever use 100% of the tool diameter in the length of the cut/stepdown. So if I have a 1/8" endmill, I only want to have 1/8" LOC (length of the cut, or flute length) to get the most rigidity I can from it. Similarly, the flutes are also super important; the fewer flutes you have, the less stable the tool is and can be prone to deflection; however, the more flutes you have, the less chip evacuation you will get. So there is a balance you want, and he uses and suggests three flute endmills. It provides enough space for chips to evacuate with air coolant properly, and still have enough rigidity to prevent deflection.