I'm going to disagree with the other posters. I would go with the 720p set and save some cash. The majority of the TV people/experts/etc are correct IMO in saying that typically people cannot distinguish the difference between a 720p signal and a 1080p signal. Keep in mind that this is in terms of resolution and detail.... There are those who think there is a difference between 720p and 1080p sets, but the difference has more to do with black level improvements and processing improvements on the 1080p sets over the 720p sets rather than the actual difference in resolution.
I thought that I was able to tell between 1080p / 720p as my friends and my 1080p LCD sets looked noticeably sharper than a family members 720p Panaonic X1. This may be due to the fact that LCDs are generally a touch sharper, and that the x1 has a bit of a softer type plasma image. However, I just got a new 42c450 plasma - 720p - and to my eye it is equally as sharp as the 1080p LCD (thats a bigger screen even) in the family room (550 samsung). Bottom line is that there are much more factors that go into 720p vs. 1080p sets that affect PQ (brand, black level, processing, etc) than pure resolution/ # of pixels.