I looked at the AZ Wind Sun FAQ
. Their objection to GEL cells is primarily that they charge at a slower rate. So what? All wet batteries and every other type I know of with the possible exception of Lithium Ion will benefit from slow charging. The whole concept of a deep cycle battery is that it's designed for a long discharge period and you need to allow a long charging cycle. In a car/Off road vehicle/Class A RV where you're cranking an engine over, running a winch, using an inverter to supply AC voltage to a washing machine then it's a bigger issue.
They also mention that in really hot climates the GEL electrolyte can lose moisture and fail due to voids. Again, heat is the enemy of all batteries. They sell batteries but don't mention in the FAQ the cost differential of GEL and AGM. Even if they last 20% longer when abused (there's no evidence they won't last as long or longer with typical treatment, my experience has been longer) the AGM aren't as cost efficient. AGM does have a big weight advantage. If you're doubling the number of batteries and using the biggest ones you can find then that's an issue.
With two 6V batteries in series if one cell dies (the usual way a battery croaks) you're dead in the water. With two 12V
batteries you're back to the capacity of one battery. Charging two batteries in parallel can be a bit tricky. A battery isolator, A/B/A+B is probably a really good idea.