Check Basic to Advanced Battery Information from Battery University
. This and other sites I've been researching say AGM batteries do have a greater capability to discharge to a lower rate than lead acid with less impact on life. You have to get something for the much higher price!
"AGM offers a depth-of-discharge of 80 percent; the flooded, on the other hand, is specified at 50 percent DoD to attain the same cycle life."
Consistently discharging to the max DoD, whether 50% for flooded or 80% for AGM, will shorten the battery life compared to lesser levels of discharge. There are other considerations for battery life too though, as others have said.