Littmanns are excellent and expensive.
GrXs are excellent and inexpensive,(http://grxmedical.com/stethoscopes.asp
), I have one and it's great.
Single tubing has just that, one tube running from the diaphram/bell to the earpieces.
Double tubing is more sensitive. The outer single tube contains two separate tubes. Each of the inner tubes runs to an ear piece.
The key to stethoscopes is practice. Listen to everybody you can. Know normal breath sounds. That way, adventitious sounds jump right out at you. Also practice heart sounds. Have your friends sit/lie in different positions as you listen. You'll learn about placement to pick up the sounds.