I'm looking at a Duracell Ultra BCI Group 27M battery at Batteries Plus Bulbs. Does this sound reasonable?
It is made by Deka, so there's that, but no, those automotive 12V form factors need to see the cycles vs DoD charts, otherwise assume "dual purpose" glorified starters.
So, not nearly as good as 2x GC2s, 4 would be even better, if say running a fridge for days at a time, or bilge pumps, windlass etc at all often.
So, used for what loads, how long? Really, AH per day is what you need to know.
How long away from shore power? How often? What other charge sources while you're out?
Starting is no load at all, you're right, it's often not worth carrying a big heavy batt dedicated to just that function, any decent House bank can crank no problem.
But you **do** want to ensure you're never unable to start, LVD or backup jumpstarter pack.
If that is one bank in parallel (most efficient), then really the batteries should match, buy them all at the same time, or the older one will bring down overall performance and longevity.