I would try to replicate what the previous contractor did so things would look uniform on the interior and exterior.

Installing inserts in the frames of Windows from that era doesn't always work the best, hard to say without seeing.

May wanna try to correct the communication issues and go with same Installer.

While I disagree with the previous contracts potential rationale, Todd is correct in that you will want to remain consistent so as to keep the exterior facade the same.

I'd agree with that as well. In this case, you will want to maintain uniformity. Todd's point on the fact that 15 year old homes are often not realistic candidates for a pocket replacement is very valid as well due to the type of products and installations used in that period.

More generally speaking to your question though, the factors that would point to a full frame over pocket install are:

1) No option due to existing window style (ie: if existing windows are new construction vinyl)

2) Existing water leakage or questionable original installation

3) Substantial rot to the frame (not just the exterior trim pieces)

4) If light/visible glass is at a premium. This is common on homes that have few, or smaller windows to start with.