Ooh, tough one, particularly for the IS. For me, optimum is 10 points, but sometimes I have to accept less than optimum to get the other equipment I want, with the IS standard (heh) of 9 points being my next go to.
Although not included in the poll, sub-7 point values aren't completely without merit, although this often depends upon other features such as a specialization that reduces direct combat, very long ranged weaponry or stealthy armor.