For all the time s i've been to Dianna's - i've been moderately impressed. The issue i really find is that they don't necessarily have the best seafood. But that's only if you're comparing it to organic and wild. If you're comparing Diana's to your local store, or T&T, then Diana's is better by a landslide.
The difference in price is roughly +$2/lb at Diana's, or +$3/lb at a wild/organic store.
The difference in quality between the three is tremendous. Primarily, as you go up the price range, you get less fattier fish, and fish with more twitch muscles. At Diana's, you're sort of at the half way point.
Knowing this, you have a great selection at Diana's, average fish cutting, and knowledgable staff with just above average seafood quality.