The composition settings are not going to make a difference to your final product. You need to know what format the broadcast station requires, and that is the format you export your final output to. The composition settings for the duration of the work done to the project can depend on a whole range of factors - generally, from what I have seen, HDTV - 1080 25 (for my region of the planet that is!)
If you live in NTSC land then you should work in HD at 29.97 fps. If you work in PAL land then you should work in HD at 25 fps. In both regions the master composition should be 1920 X 1080 pixels.
For delivery you must talk to the broadcaster. Networks and TV stations are very picky about what format you use for delivery. They are very picky about what codecs are used, what the audio is set to, and what length a spot is. For example, in the US, the audio must be 28 seconds long for a 30 second spot.
As the others said, ask your TV station or local cinema for what spec they want and then create your content accordingly based on AE's presets.
I suggest you work in a 16 bit image sequence format (TIFF, EXR...) or 10-bit (DPX) throughout the multiple steps of editing, color correcting, etc. DPX 10 bit is also a standard format for film delivery (and more and more 16 bit TIFF).
When your video is final and ready to be delivered:
Quicktime ProRes and Quicktime DnxHD 1080p are known as the best format for television and broadcast. 24p (23,397 or something...) is my prefered frame rate, but NTSC is more common I think (1920x1080 29,97fps progressive).
The suggestions above are helpful, but they may not necessarily be correct.
Here's the ONLY way you're going to know for sure: find out the delivery specifications for your project and work backwards from there. You have to contact the ad distributor / broadcaster / advertising agency / editor.... whoever receives your work. Presumably, these people already know what they need from you. You need to find that out.
If you don't do that, you're only wasting your time.
Dave makes an excellent point, which was also made by others: Begin with the specifications from your client and work toward those.
Read this article, and be sure to follow the link to the article by Aharon Rabinowitz toward the top:
These are great suggestions if you work for a TV station. I produce national and international press releases that will be played on a thousand DIFFERENT TV stations. I just need to know a good, uncompressed format.
DNxHD for some
ProRes for others
Some may require AVI with a specific codec...
Oh -- VNR's. How do they get distributed?
If they truly go international, you'll probably need 25 and 29.97 versions... and if you're distributing via FTP -- a common method -- I can tell you right not that NO news department using VNR's has the time to download uncompressed video!
I happen to work at a Midwestern CBS / Fox affiliate owned by Sinclair Broadcasting. The most common form of download I see is mp4/h.264 at 1920x1080 29.97.