You can only pack so much lead and acid into a Size 27 or Size 31 case, and the physics of a lead acid battery are the same whether made by Lifeline, Trojan, or Excide. You can make a battery cheaper by leaving out some lead plates, but once you fill it to max capacity there isn't much else to improve upon.
AGM batteries have long useful lives, especially when compared to flooded cells that do not receive adequate care, but they don't yield any more AH per lb of lead than another type.
As John says, read the spec sheet when comparing batteries. Use the 20 hour rate for AH, or the RC (Reserve Capacity) minutes value to compare. Those are standardized measurements you can rely on. Forget the sometimes exaggerated claims on the ads.