It matters more what they say and not who they are. Sure it's nice to have some great letter from a well known prof but that's pretty rare to get. Make sure it is someone you know well and you think would have very good things to say about you. Also, make sure you ask IF they can write you a nice one. They can write a letter for you but say in it that they can't support you.

Whatever you have to do to be happy is always worth it!! Good luck!