WITH RECURSIVE or double scan?

-- image column is a JSONB containing something like
-- {
--   "key": "16c536a1-b552-4e13-b171-2f5370e4627f",
--   "width": 367,
--   "height": 239,
--   "length": 48910,
--   "extension": ".png",
--   "timestamp": "1563904541.1229434",
--   "thumbnails": [
--     [
--       340,
--       221,
--       0.92643,
--       {
--         "key": "7203e420-cb1e-457d-bd70-01412137a2e5",
--         "width": 340,
--         "height": 221,
--         "length": 9081,
--         "extension": ".jpg",
--         "timestamp": "1563944400.5420659",
--         "content_type": "image/jpeg",
--         "reproducible": false
--       }
--     ],
--     [
--       300,
--       200,
--       0.92643,
--       {
--         "key": "2f80bf8c-07bb-11ea-b414-3085a99ccac7",
--         "width": 300,
--         "height": 200,
--         "length": 8025,
--         "extension": ".jpg",
--         "timestamp": "1563944400.5420659",
--         "content_type": "image/jpeg",
--         "reproducible": false
--       }
--     ]
--   ],
--   "content_type": "image/png",
--   "reproducible": false,
--   "original_filename": "/tmp/tmpoqwnnn3x/emblems/logo.jpg"
-- }

CREATE TABLE mytable (id integer PRIMARY KEY, logo jsonb);
INSERT INTO mytable (id, logo) VALUES (1, '{"key": "16c536a1-b552-4e13-b171-2f5370e4627f", "width": 367, "height": 239, "length": 48910, "extension": ".png", "timestamp": "1563904541.1229434", "thumbnails": [[340, 221, 0.92643, {"key": "7203e420-cb1e-457d-bd70-01412137a2e5", "width": 340, "height": 221, "length": 9081, "extension": ".jpg", "timestamp": "1563944400.5420659", "content_type": "image/jpeg", "reproducible": false}], [300, 200, 0.92643, {"key": "2f80bf8c-07bb-11ea-b414-3085a99ccac7", "width": 300, "height": 200, "length": 8025, "extension": ".jpg", "timestamp": "1563944400.5420659", "content_type": "image/jpeg", "reproducible": false}]], "content_type": "image/png", "reproducible": false, "original_filename": "/tmp/tmpoqwnnn3x/emblems/logo.jpg"}');

-- I wanna get the list of all "key", of both the "master" images and their "thumbnails"

-- First approach, using a recursive CTE
WITH RECURSIVE
  logos(i) AS (
    SELECT t.logo::jsonb
      FROM mytable AS t
     WHERE t.logo IS NOT NULL

    UNION ALL

    SELECT jsonb_array_elements(i->'thumbnails')->3 FROM logos
  )
SELECT i->'key' FROM logos;

-- Alternative, a union between two distinct table scans
SELECT t.logo->'key' FROM mytable AS t WHERE t.logo IS NOT NULL

UNION ALL

SELECT jsonb_array_elements(t.logo->'thumbnails')->3->'key' FROM mytable AS t WHERE t.logo IS NOT NULL;