Powder Coating will hold up better but its going to cost a good bit to get it done and if you ever chip or scratch the hell out of your rims while 4wheeling around rocks and shit your going to have to have them re-coated which is just $$$$ down the drain.
Now with spray can you can Re-spray them anytime you want for around $2-3 and it will last just fine if your just driving around town and for 4wheeling you can keep a can in your truck and respray the rims if they get scarred up. To degrease get a simple can the stuff from Kragen or Autozone and spray the rims then spray it off w/ a hose or take it to a car wash and spray it w/ a pressure washer. Then dry the wheels complete and let them dry, then go ahead and start painting.
If I were to do it, I'd go w/ the rattle can until I got some Wheels that were Black in color from the beginning instead of wasting the money on powder coating stock wheels.