As techtre2003 said above, the 3D 720p projector has a certain fun factor, but it wears off pretty quickly. It's great for 3D BD movies and 3D gaming (I used Xbox360 and PC games via Nvidia 3D Vision and also through a 3DHD Avermedia HDMI card) but for 2D movie watching, it's "ok", not as good as a 1080p PJ. You'll get used to the quality, but IMHO, the 2D on the 720p projector is no where near the quality of the 1080p PJ. As a matter of fact, my GT720 had "screen door" effect on my 150" screen, visible from 12 feet away. I know screen door effect is supposed to be common on LCD projectors, not DLP projectors, but not what I experienced. My Epson 8100 does not have SDE and has been great for the 1 year since I bought it. I stopped using The GT720, due to several stuck pixels that started to appear all over.
If you could later, get both. But if you can only afford one, get the 1080p. IMHO, I would get nothing less than the Epson 8350 or a Epson 8100, as these are very inexpensive and have horizontal and vertical adjustment. I personally would encourage anyone that is in the market for a home theater to continually save their $$$ to get the best PJ they can buy, as opposed to settling for what they can currently afford now. Myself, I'm saving up for the new Panasonic AE7000U 3D LCD that will be coming out next month. This PJ will replace my Epson 8100 and Optoma GT720.