The shop doesn't matter as much as the trip leader. Some shops use the trip leader spot as a "reward" for something, I'm not sure what, because a good trip leader can make or break your expeience. The boat is what it is, and a shop won't get you better or worse service than another shop. Your trip leader, if he/she is an experienced Flower Gardens trip leader will have a great save a dive kit (the boat carries minimal save-a-dive), will know how fix or cobble up your broke stuff, and dive with those who need it. A poor trip leader will take pictures and ignore the customers that have paid for them to be there. after running 250 or so trips to the Flower Gardens, I definitely have shops.trip leaders that do a great job, and those who are there for their "reward". The trip leader job is (when done correctly) one of the hardest jobs on the boat.
If I were paying for a trip on the Fling, I'd look for a smaller shop, like Island Divers in Galveston that only run a few trips per year. You're likely to get the shop owner or instructor that has a lot of experience in the Flower Gardens and knows how to look after you. They may be teaching an advanced course or specialties which you may or may not choose to jump in on.
What are your personal skills? Do you need looking after, or are you self sufficient? Can you listen to boat briefings, or are you smarter than the folks who run trips there year after year after year? Have you serviced your gear and tested it in the pool after servicing, or is the green stuff in the filter just another reason to remove the filter? A lot of your experience in the Flower Gardens will depend on your own comfort diving, comfort on boats, and comfort in your gear. I'd call the three shops that have the time frame you are looking for. Ask who the trip leader will be and chat with them. It's just like looking for an instructor, your success depends on how much pre-trip effort you are willing to spend.