If you buy a second battery you will have to continue to maintain the charge on the AGM (Absorbed Glass Mat) battery, because it will discharge just sitting. It won't self discharge as fast as a flooded battery, but it will self discharge from local action within the cells just the same. If you don't, it will get past the point where is will take a charge, at it will be junk. Flooded batteries are more forgivable, and can, to a point, come back from a discharged state better than an AGM type.
Unless you have a need for a second battery, just keep what you have, and if the car sits unused for more than about 4-5 months, keep it topped off by using a battery charger (maintainer) that is compatible with AGM batteries. AGM batteries don't vent unless they're are exposed to a too high finish voltage (like what flooded cells need). AGM batteries ARE lead acid, but instead of venting hydrogen and oxygen to the atmosphere, they recombine within the cells to form water again. That's why they don't need watering during their normal life. They only vent when internal pressure exceeds about 2.5 PSI depending on the lead alloy type of the plates. Because flooded batteries are vented and the caps can be removed to check water levels, you can also take specific gravity readings that tell you state of each cell. Flooded batteries last a long time when properly cared for.
When I buy a battery, I always get the highest capacity (CCA or cold crank amps) available in the size needed. It will last the longest because as it ages it goes down in capacity, (they all do) its service life is longer because there is more to start with (pun intended).