I didn't even think about this. We are getting a contoured one.
I'm assuming it's harder for a baby to accidentally roll out of the contoured one. I don't plan on leaving the baby unattended on the changing pad (as I'm sure most parents wouldn't), so I don't think it matters anyway.
We got a four-sided contour one. For me it wasn't as much about baby rolling...but as to contain any possible "mess" that might want to start rolling. I know there is a cover on it, but that won't alway s soak up everything.
Re: Changing pads - contoured vs. flat
I didn't even think about this. We are getting a contoured one.
I'm assuming it's harder for a baby to accidentally roll out of the contoured one. I don't plan on leaving the baby unattended on the changing pad (as I'm sure most parents wouldn't), so I don't think it matters anyway.