Mine has always gone on pads. Pads in holders, not in holders, with walls around and without. lol I have the pads now in a long try about 39 long by 15 deep. I am going to give her a week or two on that and if she is consistent I am going to change the pads in the tray out with real grass sod. The sod is MUCH cheaper than pads and I should be able to rinse it out every so often and replace when it dies or goes stinky.
Forgot to say, the reasoning behind my wanting to go to grass instead of pads (outside of price) is that I think it will help with any confusion she may have with throw rugs and carpet vs pads since grass is a different surface all together.