JS,
AGM batteries (Optima and Seavolt) have many attributes that make them superior to standard flooded lead acid batteries.
Btw, a West Marine Seavolt is a Deka battery which makes an excellent product.
However, most elect to go with a standard lead acid battery like the Interstate deep cycle (27 or 29) because of the greater dollar to amp/hour ratio. Tough to beat the old standard in that respect.
If going with a single stereo battery then definitely step up to a group 29 or 31 for the moderate size and investment difference. Go strictly with deep cycle.

David