Royal Canin MINI Puppy is a great brand! I've been seeing it around this forum a buch of times. I barely got myself a bag on Tuesday and my pup loves it! Plus, the pieces are small! Like a 1/4 of a dime, I would say. It's like $9 for a 3 lb. bag. I never heard of the brand you are feeding your chi though, so I really can't compare. Royal Canin also has it's own separate food for Chihuahuas once they reach their adult age! Hope this helps a little bit!
I don't know about Bil Jac but Lily is starting to love the Royal Canin Small Breed (nice tiny pellets). I've also ordered a bag of the RC Chihuahua specific, which I couldn't find it at my Petco or Petsmart, and I'll try her on that.
I love the crunching sounds she makes when she eats her dry food. It's like music to my ears.