Powdercoating gives a better finish, but typically costs $50-125 per wheel. And, you need to remove and re-mount the tires, which can be $$ if you're putting the same tires back on.
Painting can look pretty good if you do it right, and costs probably about $30 for enough stuff for 4 wheels.
The reason a lot of painted wheels look bad is because the person didn't take the time. Mine look good, because I thoroughly removed all the old paint from the wheels, sanded them down, painted several basecoats, and several clearcoats. A lot of people just scuff up the wheel with sandpaper, then do 1-2 basecoats, and call it a day.
