For Postgres, there's a strong case to be made VARCHAR(n) is preferable to TEXT+check constraint. As you note, they're stored the same on disk. But for a varchar, you can increase the length with just this:

    ALTER TABLE a ALTER COLUMN b TYPE VARCHAR(n)

This only requires ACCESS EXCLUSIVE for a fraction of a second while the metadata is updated.

Whereas updating a CHECK constraint will require a full scan of the table. If you do it naively, this locks the table for the entire scan. If you want to avoid that, it's three steps:

    ALTER TABLE a
        DROP CONSTRAINT b_c,
        ADD CONSTRAINT b_c CHECK (length(b) < n) NOT VALID;
    COMMIT;
    ALTER TABLE a
        VALIDATE CONSTRAINT b_c;

So as long as you're only increasing the length (which in my own experience is a safe assumption), VARCHAR is much easier to work with.