I think you need to very carefully compare the overflow/weir construction, and the way that is integrated into the top lip of the tank.
All these guys can make an acrylic box. The good tank builders differentiate themselves with the craftsmanship that goes into the overflow boxes, weirs. Also, there's the stand--mine came with a very high quality, thick, custom cut piece of heavy plywood for the tank to sit on. Buying and cutting that myself would have been a project, and expensive. Be sure the builder does that for you.
And the sump. I had mine built custom along with the tank, and the way it was built saved me a flood in the house while I was outta town. My filter socks clogged with gunk from new driftwood.
This is a very long term purchase. I would not compare price, I'd compare quality. Saving a couple hundred bucks right now is not a big deal for a tank that you'll probably have for a decade or more.
Ask around "who is the highest quality custom acrylic builder". That's what I'd do.