CB has an 11yr old AGM that is still working like new 
It's Gel technology but there is an AGM now sitting next to it. I've run gels since the mid 90s (I have had four) and they work as combo style (start/house). Two on board and rotate on each outing. I find that the gels are expensive and don't have the same cranking amps as similar size flooded or AGMs but when taken care of, last a long time. Fact is, most batteries live shortened lives due to owner neglect more than anything else.
Yamaha (Japan) still specs lead acid batteries on their outboards. My old 91 has the typical (for its time) charging system and I've seen it get up around 15 volts +/- on my gels. I always read they were sensitive to overcharging so I've switched to AGM (one at a time). The word is they are not as sensitive to the older style outboard systems. The overcharge theory is that when you do overcharge a lead acid, you loose the electrolyte solution. Those type batteries allow one to add more water (and I'd guess acid if needed) where as the sealed style (AGM and Gels) vent the solution and it can't be replaced. So the key is to not overcharge.
Same can be said for failure to maintain a sufficient charge. Allowing a lead acid to completely discharge is often a death sentence. Gels not so much and AGMs, well they say you can but bringing either back can be tricky. Again, maintaining the charge over time is the key. I never (well once here lately) let my gels fall below 75% charge before recharging. I go out to the boat, flip the battery switch to 1 then 2 and read the percent charge on a gauge I had installed just for that purpose. Once it reached that percent discharge, on went the charger (yes a charger designed for gels). IMHO, that's the key to long life.
As far as 12,24,36, 48 I can't speak. As stated, I run 12 volts at a time so I'm not looking for a different charger but do look for a specific type (gel/AGM).
Good luck.
PS... I charge my flooded (car/truck/motorcycle/lawn tractor) with the same smart charger as I use on the AGM and the gel. It claims to charge them all and apparently it does. Obviously, it's not a permanent mount in the boat.
