我老公帮我改了一下，我觉得不错，现奉上以作为范本如有需要者。

Dear Dr.xxx ,

I really appreciate the opportunity to work with you at the XXX on a topic that we are both interested. I have been offered a xxx position at the XXXX on several projects using XXX. After long consideration, I have decided to accept the offer at XXXX as I continue to search for a XXXX position.

I would, however, be very happy to collaborate with you on future projects.