Personally, i've experienced a pretty big difference between the two.
In fact, just received an order this week from both.
Purple Tang from DD and 5 blue reef chromis from LA.
The tang is gorgeous and healthy.
Only two of the reef chromis made the trip. And they are (were) all tiny. I knew they were sized small, but at $20 each I was hoping for something slightly more substantial. And alive.
That said, you can't beat their warranty and customer service. The woman was very apologetic and credited my account no questions asked.
Every time I order from DD, I am VERY pleased with what I receive.