Wet cell batteries can loose performance and life if not charged every 3 months. Notice I didn't say "will", I said "can", I have had car and boat batteries last for years that only got used/charged a couple times a year, your milage may vary.
The AGM batteries are supposed to retain something like 90% of their starting power after sitting for a year, while a wet battery will be down to something like 50 or 60%. Now this is with out anything connected or drawing it down. Our EHCs draw something but it's not much. My AGM doesn't start any better than the old wet Harley battery I replaced, but it will turn it over longer when it doesn't start.