How should I let a professor know that I don't want to work for her? I am interested in the research of two professors, (one more so than the other) at a university I am applying to for PhD (let's call one Ms. E - less interested and Mr. K - very interested). I met both the professors and both seemed to be interested in getting me on their research team. I had a second meeting with the Ms. E and she asked whether I was talking to any other professors. I told her about Mr. K and that was it.

Next thing I know, Mr. K under whom I was actually planning to work, emails me saying I should continue my application under Ms. E. 

Now I am confused as to how to navigate this problem. They both had planned to pull some academic strings to get me in as soon as possible (I am an international student) and both have promised me funding. I had not verbally committed to anything with either but only told them that I was interested in their research and wanted to talk to them about it further.

How should I let Ms. E know that I would rather work for Mr.K?
It isn't much clear as to why Mr. K would insist you to work with Ms. E. It might be either of the following


It might most probably of Ms. E's suggestion/influence.
They might be on the same team (@virmaior's point)
Mr. K has just got another student he prefers over you. 


To figure out why and how to react bases on why, @demitau's suggestion would be most appropriate; speaking openly with Mr. K should provide a solution.  

If the answer to the last question is what you require (how to let Ms. E know I'd rather work with Mr. K?) then the most diplomatic approach is to say something like this: state that you feel much honoured to work with her. But it seems you current research interest are much inclined to that of Mr. K. Hence the decision.