Most reviews have claimed little difference between 720p and 1080p on anything but the very largest screens, so it might not be worth paying extra for 2" less screen and the higher res.
You also need to consider the size of your room, while where you can get away with 40" you can also get away with 42" if 40" is already dominating the room you might not want to go to 42".
Also, you are paying $300 more for the 1080p set - so are you planning on investing in a lot of hi-def movies on Blu-Ray and HD-DVD or a lot of hi-def gaming? If you are just looking at broadcast Hi-Def and upscaled DVDs it is not worth an extra $300.
Plus which looks nicer? Samsung styling is normally top-notch but some of their designs are not as good, and you have to look at the thing as well!