what's the favorite replacment starter for these sixes?

You can downsize the battery too for even more weight(and cost) savings once you go mini. I went to the smallest car battery at Costco without a problem
In a given model/price point of battery they are often the same price regardless of group size. For my money I always get the biggest highest amp battery that I can make fit the tray. Especially if it's the same money! The extra capacity in a battery never hurts a thing.