How is dual core unnecessary? They have faster and better gpus... and even if you don't get to use the full 2 cores now, they still provide much greater power. My fascinate (galaxy s) has one the most powerful (if not the most powerful) singe core gpus, and jet there are times where it will have issues with some web pages.
As far as battery goes, the GS II has been getting great review so far. It is said to have no problems of getting through the day on one charge. At the end of the day, I suppose it depends on what you realty need. The majority of people would be more than happy with a single core. However I feel that investing on a single core phoone at the moment is a waste of money, especially if your going to be on contract for 2 years. Dual cores are happening now, and quad cores are right around the corner.