I'm an Assistant Editor. When working in Avid, when I have archival footage with frame rates other than the project frame rate, I typically do frame rate conversion to 23.976 in After Effects and have been happy with the results.
If I did the frame rate conversion in Premiere, would the results be identical? I don't know if Premiere does frame rate conversion in exactly the same way as After Effects.
It would depend on what frame rate you are converting from.
I assume since you are using AE then you are putting the clip on a 23.98 composition and changing the Frame Blending to 'Frame Mix' or 'Pixel Motion'?
As I understand it, the same result will be achieved in Premiere by choosing: Time Interpolation>Frame Blending or Optical Flow. Someone will no doubt correct me if I'm wrong but I believe it is the same engine so result should be identical.
If your sources are 25fps however you should probably be using the 4% slower / conform method in order to maintain optimal video quality.
Yes, it's a 'frame-for-frame' - interpreting your clip to play back at 23.976 instead of 25. The clip quality will be identical (ie. no loss, other than generational by virtue of the fact you will be exporting your clip again along with everything else on your timeline.). However, the clip will be playing back 4% slower which might be subtly noticable by you but won't be obvious to your audience. Generally speaking, if audio is really important then you will want to add a pitch adjustment because the 4% difference will alter the pitch of all audio.