Are there guidelines for choosing a 1080i signal over a 720p HD signal? Comcast offers both in my area. Does it depend on the native resolution of your tv set? Thanks for your assistance!

Each network decides which format they broadcast in, its not your choice.
For example, ABC, Fox and ESPN use 720p while most other networks use 1080i.
Most HD tuner boxes allow selection of the output format of the box though or native mode which will just output whatever the format of each channel is and your TV can convert it.

Thanks, Joxer. So would it be better to have a tv resolution that matches the signal, such as a 1080p tv for a 1080i signal? Or does the tv's native resolution not really matter?

It is always best to view material in its native resolution, however, that is not possible with today's displays. Some channels will have to be converted. It is better to buy a 1080p display that properly handles 1080i material. If you are on a tight budget, I would consider getting a 720p display instead.

Due to differences in how displays and HD set-top-boxes process digital video signals, the only way to know for sure what is the best setting is to try all possible combinations of set-top-box outputs into the display. Pick the one that looks the best.