When I first heard about the Gutenberg Navigation Block, I was hoping it would make Menu In Post obsolete, but I don’t think there is a definitive answer. Yet.
As far as I know, the Navigation Block creates menus as an unordered list of links, and can collapse them for a mobile menu, but it doesn’t (yet) give you the option of creating the menu as a select element; i.e., a drop-down, which Menu In Post will optionally do. (You can set the menu to display either as a list of links or a drop-down select.)
While it’s possible to convert an unordered list of links like the Navigation Block creates into what appears to be a drop-down menu with CSS, it would be extra work, so I guess if you’re determined to have a drop-down select element, and don’t mind adding an additional plugin to do it easily, then Menu In Post would still be the way to go.
Personally, I always prefer to accomplish things with native elements like the Gutenberg Navigation Block instead of using a plugin, so I recommend trying to make the Gutenberg Navigation Block meet your needs.