Who gets maftir a chosson by his ufruf or a person that has yarhzeit?
Before answering your question, it should be noted that both the chosson and the yahrtziet are not a chiyuv to specifically get the maftir. They both should get an aliya, but there is no obligation for any one of them to get for them to get this specific aliya. Additionally, if it is going to cause a fight, it is both the chosson and the yahrtziet to be the “mevater” and let the other one have the aliya. This is especially true regarding the yahrtziet. It is a bigger zechus and iluy for the neshoma, that the child caused shalom, and this is a bigger merit than getting the aliya with ill feeling. (This would be especially true if the other contestant for the aliya were a bar mitzva boy, who practiced preparing the haftora and the brachos, and might not be able to get a different aliya).
Having said that, if push becomes shove, the yahrtziet should be given the maftir over he chosson.
As a side point there is even an inyan that the chosson specifically get a different aliya and not the maftir. This is in order to show that he is a gadol, and his kedushim is proper, in fact for this reason in Sanz they specifically don’t give the chosson maftir.
ע מבית לוי חי”ב עמ’ י”ז שכן פ’ הרב וואזנר זצ”ל, וכ”כ בפסקי תשובות ס’ רפ”ד-ב’ וס’ קל”ו-ט.