There are cases when it could certainly be faster to acquire the block by requesting blocktxns following a received cmpctblock even though a full-block is in flight, and some where it would not be.

This could be determined by seeing how many requests are needed in the blocktxns and seeing how much of the full-block has already been downloaded. In cases where the full-block has only just started to be received (it will know how big the incoming full-block is once it has started to arrive, and also can calculate how long it will take to finish downloading), it could disconnect that connection (to abort the download) and request the blocktxns, for example.