Are photochromic lenses better than carrying two interchangeable lenses?

It depends on conditions. Specialist subreddit consensus is that photochromic lenses are convenient and 'a game changer' for variable weather, but they take a few minutes to transition and won't get as dark as a dedicated bluebird-day lens. Reviewers skiing in true whiteout often prefer a swappable low-light lens over relying on photochromic alone.