It depends on how much air you want to move and how quiet you want it. All the way from $29 Lowes inline fans, which are noisy to vortex and some of the other fans which you can spen $300 on.
Home depot has 6" 270 cfm fans for 28 and 8" 500cfm fans for 34. They are duct booster fans and are the best CFM your gonna get on a budget. They run really quiet as well.
It depends on how much air you want to move and how quiet you want it. All the way from $29 Lowes inline fans, which are noisy to vortex and some of the other fans which you can spen $300 on.
Try it both ways and see. The routing of the ducts can change the results.
But cold air flows naturally towards warm air, so sucking hot out makes more sense since the cold is already being drawn into the hood.