The problem with iTunes (Web-DL) full HD videos is that they're encoded with low video bitrate not suitable for such a resolution and this lowers the overall quality which means that a 720p Web-DL video will give u better image details (even if upscaled) than the 1080p videos.
You probably know that 1080p have about a double number of pixels comparing with 720p which will require at least 50% increase in the video bitrate to keep the same details presented with 720p videos. Apple chose to increase the bitrate only 25% which lowered the picture quality. (The numbers are just approximated)
However, animated videos don't require high video bitrates to keep the details of the image, so you'll get a better quality in 1080p web-dl animation videos than 720p (this is the only exception)
Also keep in mind that both 720p & 1080p web-dl releases have the same audio track.
To sum up: Stick with 720p web-dl videos all the way except if u're downloading animation.