I've noticed from the screenshots posted here and other places that the 1080p itunes don't appear to be that much improved over the 720p, and in some cases look worse. However, when actually watching them in motion on my 50" 1080p plasma, the 1080p files appear noticeably sharper. The other most notable improvement in the 1080p is way less banding. In comparing the same scene in 720p vs. 1080p where the 720p has noticeable banding on large areas of solid colors, this is reduced and 95% eliminated in the 1080p. This has to be a product of using better encoding settings and high profile with CABAC compression because the 1080p video bitrate is only 1000-1500 kbps higher than the 720p.
The 1080p upgrade isn't a "fall out of my chair in awe" difference, but given the choice and that there is no price difference I would recommend the 1080p download because of the encoding improvements and the overall better sharpness.
Sorry I can not post any screenshots as I have already deleted the episodes off my computer.