One factor to consider is the range under which the device will operate. 12 volts isn't usually 12 volts.. it maybe 14 volts to 10 volts, etc. You'd want to get a solution that starts near the max voltage. A lion that starts at 12v will quickly be down to 11v, etc. My SLA actually starts around 13.5-13.8.
My inverter will take up to 16v and shuts off around 10.5. So if I were going to replace the SLA, I'd really want to start around 16v. Otherwise, the voltage would drop below the min before the batteries were actually depleted. Some batteries may use a regulator but I'd be cautious about noise in that case.