You are correct, maybe the Lithium batteries will last longer, but maybe not. we don't really know!
I know from experience the AGM's should last 5 years. I think its unlikely that the Lithium's would last 12.5 years. I will base my thinking on 7-8 years for the Lithium option which makes them effectively double the cost.
Brent, I think I've tracked down my Mastervolt (Chargemaster 24/100-3) chargers and as far as I can ascertain, they are adjustable via MasterBus for a lithium charging profile.
Still need to work out about the alternator charging ability. It's all a bit difficult from the other side of the planet!