Go with a high helix >30 degrees.....prefferably 45.
Im a solid carbide kind of guy myself, but the more flutes you use..the stronger the end mill is. But also, the more flutes, the less chip clearance and they have a tendency to "gum up" quicker.
HSS Tin coated bits are pretty hard also, but they dont hold their cutting edge as long as carbide does in my experience..and carbide can be had for as cheap as HSS plain bits if you shop at the right place.
Be very very patient with copper. You WILL break alot of bits if you try to cut too much at once when in a hurry.(youre going to be breaking quite a few anyway)
When you do buy your endmills....buy about 3 times more than you think the job will require......youre going to need em'!
