Checking water levels should be about the same, whether 6v or 12v. And the price is about the same for similar amp-hour capacity. But if you don't camp off-grid, you don't need the extra capacity so you can use a cheaper, lower capacity battery.
I think you are suggesting a maintenance free 12v in the Group 24 size vs the typical flooded cell 6v GC2 battery. Yes, that is almost surely cheaper if you don't need the amp-hours. However, with that extra bit of care the GC2 will yield about 2x the useful life, so the tradeoff is there if you choose to employ it.
2004 American Tradition; 2013 Buick Verano
Homebase in the Ocala Nat'l Forest near Ocala, FL