Depending on how you will be using the shop, will depend on if there would be an issue. When I installed mine, I was advised that if I would be painting then the heater needs to draw fresh air from the outside and not the attic.
I vented through the side wall. Had the same thought as you about the vent looking ugly coming out. To be honest, You really don't notice it. I take it from your picture that the garage is not the front/street side of your house and would look fine coming out the wall.