I would vote bag.  If you make a drive over pile, make sure it is plenty moist, like close to 70%, but skip the idea of the bales.  Just make a drive over pile where each edge comes to the ground, and cover it very well.  With your cattle numbers, I would guess it will be easier to keep the feed fresh in the bag.  

We were in Traverse City area 2 years ago this week for a family wedding.....nice area along the lake