The answer is: "The Sun Has Long Been Set" expresses joy at the end, while "Summer" focuses on time to think.
In the poem summer by Amy Lowell, after she refer to season, she also refers to the summer as “To me alone it is a time of pause”, she is using this time to reflect, to think. On the other hand, in "The Sun Has Long Been Set" by William Wordsworth is appreciating the beautiful night that stands before he and end the poem with “On such a night as this is!” meaning joy.