Trojan makes AGMs as well as flooded cells, and the amp hours vary by type as well as size. AGMs produce a bit fewer AH for a given size.
But yes, it's mostly about amp-hours, but cycles (how many charge cycles a battery can last) is an important factor for longevity. A car battery can produce the AH, but dies an early death when subjected to frequent deep charge cycles. That's where the GC2 flooded cells and most all AGMs shine.
I'm running 4x Trojan 31AGMs, Group 31 size 12v AGMs. Started their ninth year last month and still performing decently. Not like new by any means, but not too shabby either.