I didn't grow up with In and Out or Shake Shack, but in my unbiased opinion, I'm definitely Team SS.
I just got a regular burger (no cheese) with all of the toppings. They grill the bun, so it's warm and soft. The pickles have a good crunch and the onions and sauce made for a flavorful burger. The crinkle fries were fresh.
If there was any question, the custard put SS on top! I love custard, and theirs is creamy and smooth, so save room for dessert! We ordered the custard at the same time as the food and put a "hold" on it--they said to go to the pick-up window when we were ready for it. We thought that meant they wouldn't make it until we were ready to eat it, but someone went and grabbed it from somewhere--it had already been made and was a bit melted.