J., I use ONLY organic milk and I applaud you for introducing the sippy cup to get your boys off the bottle. Honestly, I introduced the sippy cup to my children (who were breastfed) at 6 months (putting water in the cups) just to get them used to the sippy cup. They didn't have any difficulties so you are on the right track. Since your boys are 12 months, it is true that they do not need to be on formula. With my third child, who had breastfeeding issues from day one, I had no choice but to supplement with formula until his first birthday. My pediatrician said that I could put my baby on whole milk from 12 months until his 2nd birthday. Initially, I mixed formula and whole milk (half and half) just until I got rid of the open can of formula. Once it was all used up, I gave my baby organic whole milk from then on. I drink 1 % but my ped said that babies need whole milk until they are 2 because of the higher fat which is necessary for continued brain development. My son is 2 but I still offer him whole (organic) milk because he has always had difficulty gaining weight, even on formula.
With all that said, keep introducing the sippy cup and simply stop using the bottle. Keep the bottle out of sight so they don't think it's still an option. One thing I had done with my son, who had to drink from a bottle, I used the Avent bottles and when he was about 6 months, I introduced the sippy spout that is the next step with the bottles. Once he got the hang of it, I stopped using the nipples altogether.
As for which milk to use, I know it's a personal choice, but Organic, in my opinion is better because the milk is healthier. Why give your baby stuff that has growth hormones, and goodness knows what else. I use the Organic Vally from Publix but any Organic milk will do. Check the prices in your store...some Organics are more pricier than others. If your boys are thirsty, they will drink from whatever you give them. Don't give up...and don't get discouraged. Also, I would suggest not giving them juice or water for awhile. Just offer milk in the sippy cup. Keep trying and since you know they will drink from the sippy cup for juice or water, they will drink the milk... they are just being stubborn. Good luck!