Black Widow since the gap in MCU releases has been long and there are other MCU projects to follow that are tied into each other that need to release so the other stuff can release. Tenet may end up being the better theater experience, but it can come out whenever, and it may just be a one-time watch for me while Black Widow has a better chance of being a movie I would want to watch multiple times to soak in all the lore.
Not to mention that Dunkirk, Interstellar, and TDKR have all been pretty disappointing theater experiences for me despite the productions being well made. I just have more faith in the MCU than Nolan at this point for a movie that entertains and sticks with me. Captain Marvel and AMATW were the only MCU films in recent memory that were just alright, while everything else has been aces.
If Tenet is as entertaining as Inception, I'd be on board, but in general his gimmicks have worn off on me.