Stick with the corded SM58. Unless you have an older crowd, you are going to have to guard a wireless with your life to keep it from being KIA almost every night.
Plus, any cordless mic under $400 or so is going to sound "hot" and "tin-y" compared to the smooth warmth of an SM58.
Just my opinion, I know many KJs that use cordless mics at shows. Of the 4-5 I know that do, however, all of them have a cordless mic with cracks and dents held together by duct tape. Plus they have a $10-15 a month battery budget.
I own a cordless Shure PG58, but don't get it anywhere close to my karaoke show. It is for DJ events only
It has lasted 4 years and looks brand new after 50 or so DJ events, but it would last about 2-3 hours before a violent death at my karaoke show.
My opinion is worth about as much as you paid for it, but that is my experience!