One thing I think needs to be kept in mind is that golf cart batteries were keeping golf carts going day after day...... way before 99% of the boats out here on sailnet and their high amp hour needs were even built. The thick plates of a storage battery compared to the thin multi plates of a starting battery, while not rocket science, have been perfected long ago in golf cart batteries.
The thing is you can't just walk into Interstate (where I buy mine) and get a real A - B comparison between 6v and 12v because the 12v equal of the 2-6v just plain doesn't exist there. Order up a 12v marine storage battery and they will hand you your 1 choice at x amp hours. Ask for 6v golf carts and you will have about 5 choices - with the x amp hours of the 12v about in the middle and the #5 at the top of the line will be a significantly more durable/amp hour battery.
To compare Apples to Apples you must go to the Rolls or other significantly higher $$$ 12v battery maker to match the amp hours in the top of the line 2-6v say from Interstate or Trojan.
So as the other 'soapboxer'
above said it's a personal preference but there are some differences.
* Edit - Sorry for the incomplete posting - My last couple replacemetns have been 4D's and 8D's are so unreasonably heavy that I was 'tunnel visioning' on 4D's. As I remember 8D's would bring up my 1-5 loosly stated rating at least a notch... but their weight !!