Suggest you go to
www.batteryuniversity.com as that will explain much of the detail about both the types of batteries and their characteristics and the variations within the specific types. A basic answer to your question is that you need to look at the performance characteristics of the battery, particularly the charge acceptance rate and the discharge cycle rates and profile. The former gives you an idea how quickly you can replace the energy used and the latter the potential life of the battery - that is the number of discharge cycles before the battery falls away and dies. The latter is important for a boat's domestic bank, but how high you need depends on your usage regime. This figure can vary by a factor of up to 3 between similar capacity batteries of the same type. For example the 95ah Exides AGMs fitted by the factory in my boat are normally used in cars with stop start systems (Jaguars and Range Rovers among others) and are cheap at around £125 each. They have a discharge cycle (to 50%) of around 500. A similar size (100ah) Lifeline is around £390 - but has more than double the discharge cycles, and therefore double the potential life. There are various ways of achieving this higher level of performance which you can read about on the site.
Which to choose depends on how you are going to use them. For example a discharge cycle on a 285ah bank like mine in daytime cruising and anchored overnight is equivalent to about 1 1/2 days usage. If I cruise for 60 days a year (how I wish!) that means 40+ discharge cycles, or just under 10% of the total potential life - so the batteries should last well over 10 years (assuming they always get back up to near fully charged after each discharge). On the other hand if I were a liveaboard with both a higher daily consumption and for more days, then having a more expensive, higher performance battery may well make sense. However there are many more issues to consider in designing a liveaboard 12v system than just the storage battery.
Difference in prices is also affected by volume production - the Exides are high volume automotive items whereas the Lifelines are low volume items with a limited market. Branding and competitive state of the market where you are buying also has an effect.
The key as in all consumer purchases is to identify your needs and research what is available to meet those needs. Inevitably you will probably end up with more than one solution - nothing is ever simple as far as boats are concerned - but at least you can rationalise your choice to yourself!
