The DC31DT in the chart at the link below is the exact same as the Duracell 31DT that the thread starter is looking at. It's an East Penn battery, exactly the same battery as the Duracell and Deka, one has Duracell stickers, the other has Deka stickers.
http://www.eastpennmanufacturing.com...Sheet-0194.pdf
Also, review the battery purpose. The dual purpose has less reserve capacity than the deep cycle but offers more MCA. Selecting the right battery all comes down to demand/purpose, and it's intended use. When you go to comparing battery specs, prices, etc. it's not as simple as so many like to make it.
The $100 Duracell that the thread starter is looking at is a good battery, and a good value....which is all he wanted to know in the first place. There's no need for him to go with an AGM, an AGM is more suited for high horsepower engines that have greater MCA demands and benefit from the flat discharge rate of the AGM type battery. I certainly wouldn't try to sell him on a Group 29 if he has the room for a 31, there are many advantages to the 31 for such little (if any) additional price.