The dealers are remarkably competitive on battery replacement prices.
AGMs (1) can discharge and charge MUCH more quickly (much lower internal resistance) and (2) are non-spillable, having recombiners for the hydrogen released (thus they do not need nor can you add water to them.) They also can withstand deep cycling much better than a flooded starting battery which will be severely damaged by even ONE full discharge and take cumulative damage if run down below 50-70% state of charge.
AGMs can take whatever current you can deliver (almost without limit) up to 2.4V/Cell, which is 14.4V. HOWEVER, you must NOT exceed 14.4V; once you hit that you MUST hold voltage there and allow current to drop (absorption.) Float voltage should be 2.2-2.3v/cell, or 13.2-13.8v.
You'll notice that this is the NORMAL output for a properly-working alternator and regulator; ergo, AGM is perfectly fine in combination with them. The ONE final consideration is that if the battery reaches 120F, ever, charging should be discontinued. No automotive system I'm aware of has a temperature probe but most high-performance "bench" charging systems do.
The problem is that if overcharged, EVER, an AGM will vent since the recombiner's capacity is overwhelmed, pressure rises, and the vent opens to prevent the battery from exploding. IF that happens you're screwed because there's no way to make up the lost water. This should NOT be a concern when it comes to an alternator that is properly working BUT if you have any sort of loose ground or the regulator gets even mildly out-of-whack without an actual voltmeter (e.g. only an "idiot light") you'll never know and it will destroy an AGM quite quickly where all you'll notice with a flooded battery is that it is "using" water. BTW if you let a flooded battery level get below the top of the plates, ever, it is odds-on to internally short at that point and then it's screwed too -- so if you're not paying attention it makes no difference. Oh, and the AGM is much more expensive (about double the price.)
In applications where you need VERY fast charging (well over 1C is perfectly permissible if you can actually deliver the current), you have a regulator that will hold voltage to 14.4V and the battery doesn't go over 120F AGMs are the clear superior choice. They can deliver FAR more current than a flooded cell and thus in very cold weather where cranking amp requirements go WAY up due to cold oil in the engine they'll start the car where others will fail. But they're more expensive (by a lot) and do not tolerate abuse at all.
If I lived where temperatures regularly went below 0F I'd run them in my vehicles because the extra current delivery without voltage sag (and not a little either) would be worth the money. Ditto if I had a need to discharge to 20% of remaining capacity; a flooded battery will be ruined by even a few of those cycles. But I don't live in such a place nor do I have a need to materially discharge my starting batteries in my vehicles, so I don't see the value in that application.
Now on my smaller boat, where I want to sit at the sandbar with the stereo going for a couple of hours, well, different story. There I DO run it down under 50% of remaining charge, I DO want fast recharge when I do that as soon as I crank up the engines, and in addition the non-spillable nature of them is desirable too. In that application I pay the money.
