How do I turn the acceptance offer down gently?
Before you turn it down, you should tell the school that offered you only half tuition that you have an offer from the another school (its OK to name it) that offers you full tuition. Mention that you prefer the offer from them (i.e., the half-tuition school) but that it will be financially hard for you to make it work.

There's a chance that the school that offered you a smaller package will be able to match the other in order to get you. If not, they'll understand why you eventually turn down their offer and they won't hold it against you.