either. However it is not a float charger so you can't leave it on the batteries indefinitely. You need a volt meter of some kind to determine the state of charge.
"older model charger." is a bit vague.
Does it have an ammeter on the front to show current? Does it start at a high reading, then trail off to zero after a few hours of charge?
To provide maximum life from a deep cycle batteries, you use a different charging cycle that with a cranking battery. I would suggest you pick up one of the "smart" chargers (made by Guest, Minn Kota, etc.) designed specifically for deep cycles.
You should use 6 amps. If it is a group 27 or larger, then even a 10 amp charger will not fully recharge over night. When we fish from Dark to 1400 then plug in a 10 automatic charger the battery is still not fully charged by the next morning.
Many older 2 / 6 amp chargers are manual chargers an can over charge if you do not watch it closely.
Most newer charger are Automatic or Smart and will either taper to zero or shut off when the battery is fully charged.
