Well, you have a very nice generator
All else being equal, the 6v batteries (like Trojan T-105's) are generally considered a better choice because they are true deep cycle batteries and you can get more "bang for your buck" and more durability.
I have no direct experience with Optimas. Some people swear by them, and others say their construction isn't as sturdy, and they have lower amp hours for the space they take up. That's just what I've heard.
I think you may have it backwards concerning Optima yellow tops and charge rates. They are AGM's and you can charge them faster than standard flooded cell batteries. They'll take a charge about 2x as fast if you have that much juice (but there's no requirement to charge them that quickly).
Sounds like you want the most power you can fit into your battery compartment, right? I'd say measure the space you have and shop based on that. Get the most amp hours you can. That may mean 2 6v, or one really huge 12v.
If you go with AGMs you won't have to worry about battery height being too high to check water, etc.
Here's a link
to some more info we have posted on our web site about batteries.