There are pro's and cons to both arrangements.
First: I like DEEP CYCLE batteries for house use and this generally excludes most easily obtained 12 volt batteries.. The exception is those made for Golf car use. And of the batteries made for golf car use the most popular and thus the lEAST expensive is the common GC-2 SIX volt, put two of 'em in series and you have 230 amp hours of which you can safely use half.
With Marine/Deep cycle you should not run 'em down that far.
Now to the 1 big or 2 smaller in parallel....
One big battery means fewer connections, less maintenance, and likely less internal resistance (Higher peak current).
However it also means that if that battery fails, You are SOL till you get another.
Two in parallel, As I said elsewhere, there is a danger of a cell shorting and between the converter and the other battery it pushes enough current into the shorted battery that bad things happen.. That said.. I run 3 Group 27s and a pair of GC-2's all in parallel and one of the 27's shorted,, nothing bad happened (Save I had very little reserve power).
So.. Once I identified the bad battery, I simply isolated it till I could replace iut.. And I still had 12 volt battery power.. (About 400 amp hours worth plus change) Now I'm back up to 500.
