I would suggest first, before any conversion, to find out the delivery format the broadcast is asking for. Each station has its own specification sheet. Then, you would be out of guessing.

To point out the obvious: if the delivery is at 30p, then it's probably a good idea to shoot at 30p.

Your delivery requirement is most likely 29.97i. Since you are editing in premiere, doesn't premiere add the 3:2 pulldown if you add 24p footage to a 30i sequence?

My recommendation is to edit in 24p - that way all your effects are rendered in 24p and match the cadence of your footage. Assuming premiere can do the conversion internally, after you have edited the 24p version of the commercial, create a 29.97i (30i) sequence and drop the 24p timeline into it.

If premiere does not do it, then export a quicktime and re-import into AE. This might be easier than sending to AE directly through Premiere, since you don't necessarily want to send clips - but I'm not an AE specialist, so please correct me if I'm wrong.

In AE, create a 29.97i timeline and drop the 24P QT into the timeline. Make the movie (cmd-opt-/, I think). There are a couple of settings. Under Render settings, set your field order to the correct one - Prores 1920x1080 is upper field. 3:2 pulldown should be on and choose any of the cadences. I usually use WWWSS since that means you get 3 progressive frames first.

Under Output Module, click on format options and change it to Prores, if you are on a Mac... or whatever codec you want, really.

Make sure to turn on the audio output, or you'll have to create a new QT with the audio attached. Export he file. And make sure you play it down in premiere out to a broadcast display to check everything.

I also recommend adding a broadcast safe filter at some point in the process (either before or after you send to after effects).

Being a fan of the 24p look, I always choose to shoot 24p and add pulldown after. Films, many TV shows, and plenty of commercials are 24p and shown on 29.97i broadcast all the time. It's very common, and unless your TV has the stupid "frame-rate up-conversion" crap, there is a subtle feeling that differentiates it from the news, talk shows, etc... If Premier/AE won't do it, or is too complicated (I don't use them) JES Deinterlacer will add proper pulldown and make it interlaced. It's a free program. Just be sure to do it on your final delivery frame size. As far as 16:9 protected for 4:3, I choose to letterbox and pillarbox so that it shows 16:9 on a regular old TV (still a large majority in my area).