Spine is probably more important, but you can't afford to totally ignore FOC. If I decided to shoot say, 75 gr broadheads on 32" aluminum arrows out of my bow, I could go with a 2020 shaft or a 2312 for extreme spread on arrow weight and be spined right, but wouldn't shoot worth a hoot.

With the 2312, using 4" vanes, the arrow would weigh 464 gr and my FOC would be 6.6%. Way too low, IMO. Going to 4" feathers would drop arrow weight to 447 gr and raise FOC to 8.5%. Still low, but better.

But that 2020 now... With vanes it would weigh 563 gr and have an FOC of 4.5%. Even with the feathers it would only make 546 gr and 6%.

The higher your FOC, the more stable the arrow is. Ever messed up and shot an arrow without a point on it? It goes hog wild. Ever thrown darts? They've got a very heavy point, so a very high FOC. Ever thrown one backwards? They almost immediately flip over, stabilize and fly point first into the dart board.

I don't doubt there's someone out there shooting very low FOC and doing just fine with well spined arrows, but anything that disrupts that arrow's stability, like a gust of wind, will throw it way off course or make it hit at an angle to it's flight path. Just like a lot of the other things that a lot of people seem to like in their hunting gear these days, it works wonderfully when everything goes right, but just one little something-or-other can go wrong and leave them scratching their head about why they missed... or worse.

If you'll notice from those numbers though, switching from vanes to feathers can raise your FOC by a point or two with little effect on spine. Reducing weight on the back end of the arrow is like adding weight to the front end. If I wanted to shoot a light arrow with a light broadhead, I'd definitely shoot feathers instead of vanes.

Just my opinion, of course.