I guess it would depend on what level of precision you would like. At 40 yards, an 8MOA dot will cover about 3.2 inches of your target while the 3MOA dot will cover a little less than 1 inch if my math is right. I'm not familiar with the Fastfire line, but would imagine that 8MOA dot would take a decent chunk of your view through the sight. Personally, I like the smaller dot. I have a 4.2 MOA triangle on my Trijicon and you can't miss it.