A progress bar can use any type of a filled sprite.
A slider cannot. A slider implies reacting to user events -- and that's the exact difference between a slider and a progress bar in NGUI. The "Simple Progress Bar" that comes with NGUI uses the slider component just in case someone wants to make it interactable -- but in your case you would use a progress bar script, not a slider script.
The slider will work, mind you -- it just won't be able to respond to events properly if it's using a 360 filled sprite. There is no code for that.