I didn't realize a 23.5 used a group 24. The boat has 27 for house, 24 for motor. There is no downside to using 27's though, right? I like the idea of identical batteries.
From what I've heard, as you said, there are few battery makers left in the Country. The ones that are left, are regulated so closely that there are not a lot of differences anymore in materials or manufacturing methodologies. At least this is what I've gleaned on car sites. But the example of the Deka batteries being sold at twice the price with a West Marine sticker vs the Durcell at Sams, is just another reminder of the boating product mark ups we all know too well.