[the pilot cock]

I've been bow hunting for 4 seasons now it it's been going great. So far I've put down 2 black bears, 2 white tail deer and a bull moose. I have purchased arrows twice in that time and I'm due for another trip to the Archery section to pick up another box. The first time out, I came home with Gold tip in 340 spine. The last time, I asked the sales guy for advice on arrows and he told me the 340 ones are wrong and he sold me 400 spine. I've had equal success with both. I think the 340's may have been slightly more accurate, or maybe my newbie enthusiasm had me practicing a little more. My bow is set between 60-70 pounds. What does the spine rating affect and what spine should I be shooting?