Thank you everyone for your advice. I'm a bit slow to reply since work has demanded my full attention lately. :)
I've decided to go with four of the GPL-6CT. Two under the hood as one bank and two in the basement as a second bank. That will give me 600 AH in total, and if I need more I can always add another pair in a third location.
I thought it best to keep all the batteries the same size, that way as they age and ultimately fail, I can mix and match between the two banks to keep similarly aged batteries in pairs. If they were dissimilar sizes then one bad battery in each bank would force me to replace all four.
