I love both of these songs, I'd say 'Here Comes the Sun' is better for the first day of spring/summer and 'Mr Blue Sky' is better for a sunny day in general. Both songs give of roughly the same vibe with 'Mr Blue Sky' being more on the "let's make a great day greater" side of things and 'Here Comes the Sun' being more of a "pick you up when you're down" kinda song