720p is a progressive signal. 1280x720=921,600 pixels on screen at a given time.
1080i is an interlaced signal, meaning only half the information is on screen at a given time. 1920*1080/2=1,036,800 pixels on screen.
So 1080i actually has more information on screen at any given point in time, but due to its interlaced nature you might get things like motion blur.
For me, it really depends on the source. My cable and video games tend to look better in 720p, but I've noticed that the majority of my Blu Ray movies look better in 1080i.
Another thing to consider is the size of your screen. For example, on a 32 inch television it is impossible to tell the difference between 720p, 1080i, and 1080p unless you're ~4.5 feet or closer to your screen.