Watching a bluray movie on a 1080p display will render true 1080p video, but watching a bluray on a 720p display, the video will be downscaled to 720p.
With dvds, watching on a 1080p display they will be upscaled, and on a 720p tv they will display in true 1080p.
So to answer your questiuon, no, they will not look the same.