There's a couple things to consider.
First off, what's inside the vehicle from the factory and why?
-some vehicles come with agm batteries from the manufacturer for better charge/discharge characteristics, extreme vibration resistance or non-spillable design. There are vehicles that the PCM's voltage regulator/charge controller are programmed around the specs and abilities of an AGM battery. Replacing with a flooded battery requires having the computer reprogrammed (at least according to dealers/guides)
-sometimes manufacturers do a bad job of picking a battery or designing a charging system. Look at certain YMs of ford fusions that chew through batteries in 3-4 years. A lot of modern vehicles are burning through their factory batteries faster than ever. I've been selling a lot of batteries for 2013-2014 vehicles this year. Other times in powersport applications, smaller batteries were used where space couldn't be made. There's nothing you can really do to get a significantly longer lifespan out of batteries in these situations. Also, there's quite a few powersport flooded batteries with no glass mat equivalent.
- you just can't extrapolate your experience from one vehicle and battery to another. Getting 8 years out of a battery on a mid 00s pickup is easy because there's comparatively low parasitic drain and a big honking alternator compared to more modern vehicles along with batteries rated at at least 50% higher cranking amps than what they actually need (diesels not necessarily included).
Next, how is the vehicle used?
- AGM batteries have a number of advantages over flooded batteries, but that doesn't necessarily meen they provide any benefit to you (at least not for the 50%+ price increase) for a daily driver.
- vehicles that sit for long periods of time, as well as vehicles that only make short trips will benefit from AGM batteries faster charging and resistance to acid stratification, as well as better deep cycling capabilities. Of course this is only to a point, they're not infallible.
- Vehicles with heavy electric drains such as winches and wheelchair lifts can benefit from faster charging and better cycling ability.
- vehicles in severe service situations such as heavy shock or vibrations, rolled vehicle/extreme angles, extreme heat or cold can see better battery lifespan out of an AGMcas well.
Holy [censored], its 4AM
in a nutshell, are AGMs better? Yes. Do you need better, or will it net you more service life for your dollar than a flood battery? Who knows. Pure Lead AGMs like oddysey and northstar tend to be the best available, with any of the standard lead calcium batteries being second.
Otherwise, for a daily driver, figure out what the average lifespan is of a standard replacement, and then decide if you will need longer service life or more reliability and if you need to spend more to get it.
Also, do you plan on keeping the vehicle or being alive for that long? People get sick of cars - they also get sick and die.