no brainer here, imo. if the agm battery lasted 2.5 years and the lithium only 3, then why spend the extra money for lithium for only 6 months additional? i vote for the deka big crank or the yuasa gyz32hl (if you're talking about touring bikes).
regarding the early failure of the lithium in your fxr, did you let the battery 'warm up' in colder weather? that's one of the downsides to the lithium battery, you have to give it a minute to wake up in the cold.
My battery (OEM Harley) just failed after 6 years. I haven't had any battery last less than that, as I keep them on a maintenance charger all the time when the bike is in the garage. I looked at what was available, quickly ruled out LiIon and was vacillating between a LiFePO4 and another AGM. LiFePO4 needs a new maintenance charger, costs silly money, and reports of them failing fairly quickly (especially HD's one!) abound. I wasn't worried about the weight saving, but I do like lots of CCA, where they do score over
normal AGM. However, on balance, the shorter reported lifespan, poor cold weather performance, and the huge cost, persuaded me to go AGM again, and I have gone for the Yuasa GYZ32HL, which has more CCA than the equivalent LiFePO4.