Hey Kosta,
7 series is much harder than 6 series and more difficult to machine - to a certain point. The thing is that it's so hard (for aluminum) that chips don't generally stick to the endmill like 6 would.
You can check out the specs, might answer your questions. BTW, that site has an amazing amount of information on different metals and alloys - it's got a buttom on my browser so I have it for quick reference.
JR