I perform in small venues (sing and play acoustic/electric guitar) through a small PA system with a monitor on stage. 

I have several Shure SM58 Microphones that have a cardioid pickup pattern. I am considering buying a new mic or two and was wondering if there are advantages for live stage use, to a super-cardioid or hypercardiod pattern in a vocal mic as opposed to the standard cardioid pattern as on the SM-58. It seems to me that off axis rejection would be desirable, so I was considering a tighter pickup pattern. 

What are advantages and disadvantages of the different pickup patterns commonly used for live performance to amplify the vocals of one singer?