I own and use both. For my handguns and rifles, I prefer 3 MOA. For scatterguns, I vastly prefer 5-8 MOA. Minority opinion, I know. Biggest advantage = Much quicker acquisition of dot for me.
Remember you're shooting a shotgun with several hundred pellets, not a single projectile. Precision with any shotgun/shell combo really ain't all that great, no matter what.
If the 8 MOA covers too much of the target, you're either shooting too far or a tiny turkey. Keep in mind, the 8 MOA covers all of 3.2" at 40 yards vs 1.2" for 3 MOA dot. And...with the FF3 you can set the light intensity to a level that makes the dot translucent. You can see right through it.
In short, both are "minute of turkey." Choose the one that you can see the best under a variety of light conditions.