720 scaled up to 1080 is not an even scale factor. There are nasty artifacts. Native 1080 will be sharper and clearer regardless the bitrate.
Bitrate matters because if it is too low, you have compression artifacts which imo is a whole lot worse than scaling from 720 to 1080. If both files were the same bitrate the larger res would be worse quality because it would be compressed more. Codec would make a difference in how efficiently they were compressed but I'm assuming the same codec was used.