Now what do I do?

My last three dogs have been exclusively fed Science Diet... plus, of course, bites of what Cliff and I are eating.  Mandy and Sadie always turned up their noses at any other brand of dog food, for instance when Hawkeye was visiting and they had access to his food.  
I'm a follower of the website Seriously Free Stuff, and often get samples of pet treats and food.  Yesterday I received a sample packet of Purina One from Walmart. I offered Iris a bite of it from my hand and she actually tasted it; that was a real surprise considering how finicky she usually acts if you try and feed her anything out of hand.  
So, I went ahead and poured the entire contents of the packet into her dish, on top of the Science Diet that was already in there.  She went to her dish and carefully ate the Purina One, making sure she didn't accidentally eat any of her regular fare, Science Diet.  
Now what should I do?  Give her the one she obviously prefers, or continue to feed her Science Diet, the brand veteranarians and pet shelters recommend?