The primary benefit of spiral cell/ sixpack design AGMsis increased vibration resistance and case strength. Rectangular AGMs case strength and vibration resistance is already much improved over flooded batteries.
The same size group AGM battery, in a rectangualr case has more lead. More plate surface, more capacity, about 25% more capacity, meaning one can leave thoselights on for longer and likely still be able to start the engine
AGM's used as starting batteries, if INadvertently deep cycled, should not be 'trickle' charged. This will tickle them to death.
Goose them( 50% depleted AGM's) with ~30% of their rated capacity( 20 hour rate) until 14.4 to 14.7v is reached, then hold that voltage until amps taper to 0.5% of capacity.
AGMs that do get deeply cycled and do not get a true full charge soon after, are not going to last very long.
AGMs which do get the promptly applied higher amp recharge to a true 100% state of charge can and will give excellent service.
Relying on the vehicle's charging system alone, to fully charge any depleted battery, is unwise.
If that is the only charging source to be used on the battery, ever, then I would recommend sticking with flooded jars with a good warranty.
If the battery is never discharged much at all in between engine starts then its charge profile makes much less difference. It is the batteries that are asked to deliver a good portion of their capacity and then are not returned to full charge promptly, which will have dissappointing lifespans, and AGMs are less tolerant of partial state of charge living, compared to flooded.
So one can spend the extra for an AGM, which could easily perform worse in their specific usage.