I don't have much practical experience with interview recording, but I know if the room acoustics have too much reverb, a simple omnidirectional mic will make it sound like you're having a conversation in the bathtub.
A cardioid pattern will still pick up some ambient sound, but you have to point it at the person talking. If you want to go all out, you can set up a microphone pair in a standard XY (or Blumlein if you have the cash for bi-directionals).
Tascam has a cheap hand-held recorder that can be set up in an XY pattern (
http://tascam.com/product/dr-07mk2/), which will maintain the stereo imaging of the ambient sounds, making it easier to let them blend into the background. Again, you'll have to be pointing it at the person making noise though. Or you can get creative and set it up perpendicular to the interview having one person predominantly on the left, the other on the right.