Opinion on accepting a grad school offer after having already accepted another offer
You have to know what you want to do. If you want to go to your 1st choice school, perhaps you can come to an agreement to have the 2nd professor as your copromotor? Or didn't you pin down your supervisor and your topic yet. In that case, just go for the first school definately. 

Take a look at your contract or contact the administration. They will be able to help you with the more technical details of resigning. 

But do what you feel you want to spend the next 4 years doing!