Question re batteries

Hi Guys.

I think I need to replace my 2 deep cycle batteries on the boat.

I can replace my current batteries (2x 75 A/H) with either of the 
following:-

2 x 12v 130 A/H batteries
OR
2 x 6v 250 A/H batteries.

Either of the combinations above will give about the same A/H capacity and

will also cost about the same price.
My question is which combination would be the better option? (if any)
2x 6V joined, or 2x 12v?

I run a 130 watt solar panel plus charging from the alternator when the 
engine is running.

Garry



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