Work backwards from your delivery specs, check your budget, repeat.
Networks generally allow only one standard. U.S. broadcasters use only 1080 59.94i or 720 59.94p. Therefore, all you got to do is examine the delivery specs and see what they are calling for. Remember that you are not limited to shooting in the delivery standard. For example, if you shoot and master to 1080 23.98p you can deliver both standards networks use without loss of quality.
There are four production standards to chose from in the U.S.:
Screen size of 1080 standards is larger than the size of 720 standards. 1080 is better suited for transfer to film because it can capture more detail.
Interlaced “i” vs. progressive “p” is also an important consideration. 23.98p frame rates will make it easier to transfer the project to film and crossconvert or downconvert to PAL.
Cost can also be a determining factor as many cameras are not able to shoot multiple standards. More expensive cameras tend to shoot 1080 size.
If delivering 720p you can use any of the four acquisition standards to shoot and post.
If delivering 1080i you can use 1080 23.98p and 1080 59.94i. You may be able to use 720 59.94p only if the network allows crossconversions (really an “upconversion” in this case).
You still can get excellent results with inexpensive cameras but you need to understand disadvantages of consumer formats. Tapes shot with more advanced cameras will give you more freedom in post.