The Apple TV is going to scale it to whatever you have the video output set to on the Apple TV (ie, 720p or 1080p if it's an Apple TV 3). If you have yours set to 1080p then there may be a slightly better looking image from an iPad 3 since it has a resolution of 2048x1536 compared to the iPad 2's 1024x768. The iPad 2 would have to be scaled up to 1080p but, the iPad 3 would be scaled down. If your Apple TV is set to 720p I bet the noticeable difference would be even less, if any at all, since both would be scaled down to 720p.
In either case, if your TV is 50" or less and you sit 10+ feet away, I doubt you would see a difference at all in either case. 50" or less sitting at least 10 feet away, the human eye can't resolve the differences between 720p and 1080p anyway.