I’m a huge fan of Concorde’s AGM and the appropriate Battery Minder. I’ve used the combination in 2 Aztecs and now the 340. Never have I had so much as a weak battery. No dealing with water/acid. No annual battery box cleanup and re-coating. Nothing. They just work. No fuss. No mess.
I like the concorde’s too. They work well, no mess, no fluid spills. They are not as tolerant of poor charging conditions (overcharging is bad) and I don’t think as recoverable if you deep discharge them. Essentially, if you manage to bake the fluid out you are done!
I think 2-3 years is plenty of use in an airplane. Then rotate them out for a build it yourself ground power unit. (plug, old battery, and small charger)