ceiling. especially if there is some sort of ventilation/openness on top. box fans typically only fan when right in front of it where as ceiling moves the air around more in my opinion. plus, I've found that ceiling fans keep flys away a little bit more!
Regarding ceiling fans, make sure to get the commercial ones
with a totally enclosed motor. Available from Grainger or FarmTek for under
$100. Household models have vent holes on the top exposing the motor windings to all of thedust that a barn generates.
Box fans in a barn are know to be a fire hazard. The dust, hay, cobwebs gets into the motor and can cause a fire.
Get a fan with a sealed motor from an Ag supply place.
These can be both ceiling or wall mounted.
I have Durafan and like it very much.
Also consider putting some insulation in your barn roof. My horse is in an old pole building with no insulation at all and the sun beating down on the metal roof heats the place up almost unbearably in summer. Neighboring building of similar design but with some insulation is much more reasonable. My horse has a ceiling fan over his stall and I'd estimate that fan is on most days, 6 months of the year, in contrast to box fans which most people put up in spring and take down in fall. This year we splurged on a couple of really good high velocity fans and the difference is amazing between those and the old junky box fans we had in the past.