Definitely on the outside. Besides all the points made above, having it on the outside allows you to easily remove it (or pull it to one side) during the cooler weather when you want a little more sunlight/heat.
Shade cloth comes in a variety of densities from 10% to 100%. That varies the amount of light that gets thru the cloth. The crops you grow and the climate in your area will determine the density you need. I'm at a high altitude in New Mexico. I grow mostly greens and vegetables and I use 60% shade cloth. Be sure to purchase the correct density for your needs and area.
Having a good fan is very helpful. I've got solar powered greenhouse fans on both of my hoophouses. Between the shade cloth, vents and the fans I can keep my hoop houses between 85-95 degrees most summer days.
