AGM vs Flooded Lead Acid Batteries

Great information from everyone! Thanks so very much! I have 4 years on my Interstate GC2 batteries (after getting only 4 on the originals) and always fill them to the bottom of the split ring barely touching. They charge at 13.4 to 13.6 as checked at the charging solenoid when running (which I had the joy of changing recently)and appear to be charging about 13.2 on shore power. All the terminals are clean and maintained and the battery box ventilated. I just checked specific gravity of each cell of the house batteries and all checked like new batteries. Perhaps I am overfilling it a bit at maintenance time causing the mess in the bottom of the box. The info I have read is mixed on AGM batteries and wanted to see how other road folks felt about them. Each time I get finished adding water to the batteries I swear my next ones will be maintenance free. I feel like these will probably last a lot longer than the original batteries, so I shouldn't have to make the decision for a while. Thanks again for all this info. Lots for me to think about.