Enfamil vs similac

My 6 week old is drinking 5 to 6 ounces every 2 to 3 hours. And yes I know that's a lot. I haven't been able to feed her just breast milk due to the amount she drinks. I've had to add formula to her diet. I've noticed that enfamil seems to be used alot more then similac. At the hospital they give it to you at the pediatrician they give it to you. What do you ladies prefer