If the offer made does excite you and you have made up your mind not to go ahead with it, make sure that while declining/rejecting the offer be polite and be gracious while informing the officials about your decision. Academia is a very small world, you might have to interact with the same professors in the future, hence never sour the relationship.

As we have already discussed, it is always advisable to be courteous even while declining an offer. Here is a template on how a rejection mail can be drafted:

Dear/Respected Professor (Name),

Words are inadequate to describe my happiness at receiving your offer of admission at (name) University for the (name) program. I am highly obliged that you have considered me worthy enough to take up this course under your esteemed guidance. However, with remorse I inform you that I cannot accept this offer. I was a very tricky and a very difficult decision to make, but I have already decided to join (name) University under the (name) program.

However, I cannot thank you enough for your patience and guidance throughout this process, and hopefully we can extend this relationship further as I am sure I will seek your help time and again in the due course.