As you said we need to tighten at the back that means stopping the counters, that's why I think Pep will prefer Jack in those big tight games.
We were quite stable in defence with Doku vs Liverpool, they had two shots on target. They scored just after Haaland missed a good chance created by Doku (and Alvarez missed another good chance created by Doku minutes before that). Thus this 'stability' point is a bit overstated. It's not like Doku loses the ball much more often or doesn't tackle or something. His pass completion rate is pretty good.
I'm happy with them both. There are trade-offs with using them, wouldn't be disappointed to see any of them in the starting XI. Nice conundrum for Pep to have.