I think this is an excellent question, provided you do not think of it as a literal comparision.
I think he is talking about the difference between body and spirit here. The discussion of whether or not he should use a nature metaphor to speak of her reveals his fundamental problem with trying to use that metaphor: everything in nature either dies or is too harsh. Her body, too, will die, and her corporal beauty is impermanent. So when he talks about summer, he could be talking about a person's body at the height of its attractiveness. When he talks about the buds of May, he may talking about the body of a young, "budding" or developing person. The sun may represent the outward beauty of a person: it is true that sometimes the sun shines too hot, (and beauty stuns or dazzles rather than entices) and sometimes it is "dimmed" (and we can't see the beauty at all. When he says that "every fair from fair sometimes declines" that might also be a reference to our bodies and the potential for us to age and become less fair than once we were. Our bodies are maddeningly mortal: they flash with youth and beauty and then they age until everything we were is eclipsed by the shade of Death (this was especially so in Shakespeare's time).
The spirit, on the other hand, lives. Her essence transcends her body, and he is capable of capturing that essence in lines of poetry; hence, though her body must die, her spirit through him, becomes eternal.
Sonnet 18 could be compared to the human body—anything can be compared with anything. The question is, how close is the comparison, and how useful is it? Along the way, you'd need to ask yourself how you are aligning the comparison. For example, does it start at the head and move…to the feet? That doesn't work, because of the centrality of the final couplet.
Does it start on the skin/surface and move to the heart? That takes care of the couplet, but gives you trouble along the way, because skin is explicitly discussed in the middle of the poem.
So, it could be compared, but I'd say, not that usefully.
If you're not going to take the lines literally you would therefore say that sonnet 18 stands for human body. The body of a lady, who is the subject of the sonnet, is being directly compared to the season. The summer, as mentioned in the sonnet, represents the beauty of the lady in its most attractive sense.Likewise, the darling buds of May speaks of her youthful beauty which is in the stage of development. Just like the weather, the beauty also changes depending on the inner spirit that one has. The couplet summarizes that the although her body fades and leaves nothing but the spirit will always remain and will never perish in his mind.