Let me preface this by saying I don't own any custom tanks. However, I've been doing some research on custom tanks, which included sending quotes to Glass Cages, Miracles In Glass and Aquarium Obsessed. Far and away GC was the cheapest, by about 30%.
However, there are a couple of things they don't do. Firstly, they don't use PPE Starphire glass. Instead, they use a different low-iron glass that some say is less clear. Also, they don't machine-grind the edges of the tank. This can lead to chipping and a loss of structural integrity. Third, their silicone job, by some accounts, sucks.
Whereas my tank from GC came in at $560 before shipping, my tank from AO came in at $1000 plus shipping, which would be $400 tops to Dallas. MIG was $1350 shipped, so it's pretty much in the same ballpark.
Lots of folks are OK with GC tanks. Hell, Thunnus' looks pretty damn nice in his pics. However, lots of other people, including some locals in my area, have had some issues with various aspects of their work. The primary complaint is that the quality of the workmanship, like the price, is cheap.
That being said, I still almost got a tank from them, and if I hadn't just blown my money on a Maristar, I would still be thinking about it, as they're coming to a show in my area and I could save roughly $300 on shipping.
My recommendation to you is that you try to find reefers in your area who have tanks from various custom manufacturers and check them out yourself. Even still, you might be making a crapshoot receiving a tank from a vendor that's far away. I'll probably end up getting a tank from AO despite the major price difference; the tank itself needs to be a high-quality, aesthetically pleasing piece of furniture that houses the living art within. Having seen the wide variety of work on his website, I know that Will at AO can provide that.
Good luck making a decision!