GC2 6V batteries at Sam's club or Costco are 2/$200 (or less, I paid $190 w/core charges and tax). You won't find a decent hybrid Marine 12V for less than $130. With 2X 6V's you'll have around 220AH of capacity. The 12V will have around 100-120AH of capacity.
What I'd suggest is to buy the Sam's club 6V's and put $200 into solar charging. At $400, you'll be at < half the price of a Honda generator, won't have to lug it around, or buy gas and oil.
If you're only going to use a battery occasionally and have a tight budget, buy one 12V Trojan or other deep cycle battery and revisit your battery/charging situation later.