IMO spending the extra for an AGM battery isn't justified for most applications.
If maintained properly, one should get 4-6 yrs service. One thing LA batteries don't like is to be left in a partial or fully discharged state. LA batteries have a low self discharge rate and can be left unused for periods of 6 months and still have a large % of charge left.
Was working on the batteries in my motorhome yesterday and have been neglecting one of the batteries as it is hard to get at. Needed a lot of water to top up as it has been on a 3 stage charger for many months. Was on a charger for about 18 hrs, did a 4A/3hrs discharge this morning and right now looks like battery is about 25% down. This battery was put into service in 02.
Keep all the connections clean and tight, keep the battery charged and you'll see a long service life.