i have space for 2 battery's .

should i use

2 12 volts ?

or

2 6 volts ?

No doubt, two 6 volts

yup

They're a bit pricey, but I've had excellent luck with 2 1200 series Odyssey batteries.

I went two six volts. Honestly I only see them being a little better.

Ditto on the 2 6volt batts

they are cheaper than most 12 volts and offer more juice.... was a no brainer for me.

2-6V

2 6v from sams I couldn't be happier

I would go with 2 6v.
Here is a good link about batteries

http://www.ccis.com/home/mnemeth/12volt/12volt.htm

I'd recommend 2 Trojan T-105s or larger if you have the $$$
http://www.trojanbattery.com/Products/Marine-RV.aspx

Do your research its easy to find and there have been many many threads on this... Unless these batteries are used for starting engines then 2 6 volt batteries is the only way to go

Two 6volts...