I've never yet seen an AGM last longer enough than a plain old battery to justify the extra price. When we bought our boat 6 years ago there was a new AGM for the genny. It went bad 3 years ago. But the already 2-years-old-when-we-bought-it engine batteries finally gave out just last year. And for what it's worth, my 2002 Ford pickup turned 10 years old last month, and is still running off the original Motorcraft battery. My 2004 Dodge Stratus is also running the original battery.
I'm sure my experience isn't typical, your battery life may vary.
But to flip the coin, my trusty old 1984 Goldwing is going to get an AGM, because I've gone thru 7 regular batteries in 7 years. If an AGM will get me thru just TWO summers, it'll be worth 3x the money!
1988 SkipperLiner 53x14
1995 Tracker Party Cruiser 32 *for sale*
2003 Chaparral 260 SSI
2000 Allegro Bus 40' DP