Most manufactures will state that some Deep cycle lead 6V batteries will get about twice the discharges etc vs and equal sized 12V be it lead, agm or gel. At the end of the day, it really depends upon how much you want to spend, what your charging system has to charge the batteries. Does not do any good to put in AGM's if you have a smallish alternator as and example, as the batteries may take more than the alternator can produce, and blow it up!
Then one has to decide how much and often the batteries will be discharged, rate etc, as to what amount of house bank one needs. I've frankly done well with a single size 24 for the last 6 yrs. But, I mostly day sail, rarely anchor
out and need upwards of 24 hrs of power, for a fridge (I have an ice chest), few lights
, running lights
, st60+ guages, and radio
. Very little power useage! Hence why I could see the OP using a single 24 as a starting battery, and another 24 in a DC style for the house battery! If he needs a bit more, there are some 24's with up to 90ahr vs the typical 70-75. Or go to a 27 with around 85-90, and some of the higher led content to 105!
This battery what you should have or not is almost as bad as anchor
option!.....well, maybe on par with sailcloth options or a bit worst.......