LIke most things, the difference is probably not significant. While having a letter from a full professor, international superstar is best (makes your application "shine"), if in fact that person doesn't know you as well as someone less ranked, then the letter is rather worthless.
Believe me, there are a lot of average letters out there and most of them come from these "superstars" who don't really know the applicant. I don't care if the letter is signed by Halstead if it seems apparent he doesn't know John Q. Applicant from a hole in the ground.
Plenty of letters are from associate or assistant professors and it doesn't really matter; the content matters. Of course, this does not translate to it being ok to have a resident or non-medical person (ie, clergy) write you a LOR.
As for letters from outside your field of choice, again, it doesn't really matter if they specify your field or just say House Officer. After all, does a Internist really know what it takes to be a Surgical resident, and vice versa?