I found the best comparisons to sonnet 73 in its surrounding sonnets, 71-74. They are all similar in some ways. The topic and theme of death and love continues throughout, even though 73 is the most unique in its comparisons to seasons and fires and stuff. 71 seems similar in that it also contains a couplet at the end that breaks iambic slightly (like 73), and that puts emphasis on the rest of the poem. (At least that's something I think 73 did.) 72 is similar, focusing on love in the last couplet, but it is in iambic. 74 may be the most similar in content, but doesn't seem to rely on the couplet as much to me.