I usually only take one horse of mine to a show. However, the last two I went to, I took two. The first time I took my mare to a show this year, I brought my gelding. However, this was a lesson for him, not for her, otherwise he'd have stayed home. I wanted him to get over being tied to the trailer while we walk away, and my mare had no real issues with this after the first 5 minutes. And actually, she wouldnt have cared at all, had he not been loud and acting up. Plus, I didnt care, I just go do fun western gaming stuff, didnt matter to me if we got ribbons or not. In the end he got over it, and was standing nicely whenever we returned for a drink after each event. I made the day a training session for him.
The second time, my mare was in season, and things were a nightmare. She was a total basket case, and he was a real jerk. He came along because a friend was riding him and he ended up throwing her off that day. We went in for keyhole, and she went past the box, to the end of the arena to yell for him. And it was everything I could do to control her. Not an experience I wish to have again.
So I would say just take the pony, unless you wish to make a training session out of the trip :)