Just picked up three of the Duracell AGM's (group 31) from Sam's Club over the weekend. I reckon they're not what the Odyssey or X2's are, but then again... they're only $180 each!
We were not even members of Sam's Club either. Been Costco members forever though. The closest battery we could find was another Duracell Platinum at Batteries Plus (Batteries and Bulbs) for $240 ($216 after 10% off). That's still $108 more than Sam's. So we joined at the $100 level, then got $10 back for the every $500 spent. Three batteries were $540 so that's already paying for itself. Going back Saturday with a bud, to get 3 for his Ranger. Don't think we'll keep the $100 membership after the first year though, but for now it's free money.
FWIW the X2 *IS* The Odyssey battery, only it's $100 cheaper. Read a story several years ago about the inventor of the Odyssey selling the company, but still held the patent. So now Batteries and Bulbs sell the X2 (and I think the one that Summit Racing sells as the XS) as basically the same (Odyssey) battery.