[scottandme]

Carbide is more rigid and can run at much higher SFPM, but your operation doesn't really need that - pocketing in 1/8" aluminum should be pretty easy, and a 1/2" HSS end mill is going to be plenty rigid for almost all operations on a manual mill. Carbide is really better suited for CNC machines with higher RPM spindles. Just stick with a larger diameter tool with as little stickout as possible (to mimimize tool deflection). HSS is more forgiving anyway if you're running the machine manually.

McMaster sells Niagara end mills, something like #2716A71 ($25) would be about right. The Lakeshore carbide end mills do have a good reputation, and are very reasonably priced for carbide tools ($48 for the ZrN coated 3 Flute EM).

Agree with Cory that using a workstop or indicating the part after you flip it should be plenty accurate anyway.