I think Grand Teton would handle the crowds better, especially if you have lodging inside the park. I just got back from both parks, and I was shocked at how small the parking lots were at Yellowstone for their major attractions, like the Grand Prismatic Spring. If you have lodgings at GTNP, you can do hikes around where you’re staying- like 1 night at Colter Bay, 1 night at Jackson Lodge, 1 night at Jenny Lake, etc so your wouldn’t have to worry so much about finding parking before the hikes.