Sorry to hear that Tufts wants you to retake Organic Chemistry--Its silly since you did well on the DAT, which proves that your C.C. course was solid. I guess that's the real sticking point. Are you willing to retake O'Chem to attend Tufts? If you are, the course should be less challenging the second time around, and will probably take up less or your time and energy. What if Tufts says it is okay to retake the lecture, without retaking the lab? Would that make the difference? If retaking O'Chem is not an option, then drop Tufts like a bad habit, accept NYU's offer and never look back.
I would definitely focus on the O'Chem dilemma first though, since it doesn't make sense to continue weighing the two schools against each other if you don't feel like you are willing to spend anymore resources/time on O'Chem, which is totally understandable.