It was so hard for me to say no to my POI. All the professors were so nice and willing to speak with me about joining their groups and directing me to students with whom I could speak with. I felt I was recruited hard to join certain labs, and I got a hell of a scholarship, so I got the feeling they really wanted me. Honestly, I know this sounds stupid, but my heart was beating so hard when I pressed send. Here is what I eventually wrote:
Dear Professor XXXXX,
I am very grateful to have been given the opportunity to work for you, and for Xschool's very generous offer. It was a very difficult decision, especially since I felt this program was such a strong research fit for my interests. However, I’ve chosen to accept an offer from Xschool. Thank you again for all your time, help and advice.
XXXXXX
One professor emailed me back, wishing me well at my other school, and said maybe we would meet in the future at conferences (my field is small).