I use the 34R. no worries about the alternator charging system / compatability with newer lead acid. Backwards compatability. Prices are about the say here, $100 for a 34R $150 for the AGM. I also use the 34R on my 560SEL, I like the transferability when the 4 to 7 year battery failure occurs. On warranty, I was taught in school that batteries have a natural usable variation at 3 sigma of 4 to 6 years. It is the typical production variation. As a result of that knowledge, the manufacturers marketing departments set up a sorting process where percentages were sorted into price vs warranty length. At the height of this silliness, I baught a Firestone guaranteed for life battery, its fouth generation freeby resides in my 70 Chevy truck.
I see now in the US, the standard warrantee is just a 2 year replacement w/no proration. The pendalum has swung. If production practices haven't changed (it seems batteries of the same # are as heavey as always), very few batteries will fail in just 2 years.