I would take Allen over Darnold.
On a similar note, but completely different question; there are rumors that Aaron Rodgers will likely not finish his career in GB, and even that he may have already, or soon will, request a trade. Some (random chatter I hear on sports radio) feel he has about 4 to 5 years left and that his skills are diminishing, though his numbers remain solid (TD to INT ratio is ridiculous - shout out to Poyer). I think it is apparent he looks better now (these past two games) since he his healthy and has mostly regained his full mobility.
Obviously this is hypothetical, which requires some assumptions, but assume Rodgers wants out of GB and does have 4 good years left (similar level of play as this season). Would you trade Allen straight up for four years of Aaron Rodgers producing at the level he is currently demonstrating he can play at? It is a complete unknown what exactly Allen will achieve, but for the sake of this question, assume he continues to progress and display signs of positive development.
I do not mean to hijack this thread, but it is all "would you rather" QB discussion, and I did not see much need for a completely new thread.