R-Day is an emotionally charged day during which you will experience the fulll gamut of emotions. From Pride, to Sorrow, to Elation, to Worry, to Exhaustion, and to Pride again. But few, if any, of those will you actually get to share with your New Cadet. You will be with him or her at the beginning, and then they are rather abruptly separated from you. The Academy has some outstanding activities and briefings, etc in which you can engage during the day, but you will probably not see your New Cadet as they shuffled through the stations and their lives irrevocably change. Some parents will mull about trying to get a glimpse of their newly shorn and uniformed child, but we were under strict orders from our son to 'stay away from the Sally Ports' and not try to see him.
You will beam with pride as the New Cadets and their Cadre march across the Plain for the Oath Ceremony, but again, you will only catch a glimpse ... and the glimpse can be unsettling and heartbreaking. Our New Cadet (NC) looked exhausted, pained, shell-shocked, and a bit PO'd (all of which were accurate according to subsequent communications with him). Still, I am very glad we went. It was helpful in helping us cut the cord of our oldest child. For him, I'm not sure it mattered one way or the other.
We also went back for March Back and A-Day (both of which can be called Parade Day, I guess ... though I assume you are talking about A Day). These were much more exciting and 'happy' times for our NC. Though again, we could not speak with him during March Back day, he did see us and the smile on his face was worth every mile we drove several times over.
A-Day is a MUST attend if there is anyway possible for you to be there. The ceremony and Corps of Cadets on parade is awe-inspiring, and the hug you will receive from your (no longer New) Cadet will take your breath away. You will have the opportunity to take him off post and let him relax, hear the stories from Beast, watch him or her eat and sleep (that's all they will initially want to do), and see glimmers of the little son or daughter you bid farewell too a month and a half before shining through the young man or woman sitting before you.
Rather long winded, I know and I am sorry for the rambling. In short: go to both if you can; but if you must choose between the two, say your farewells at the airport before Beast and try your darndest to be there for A Day.
Congrats to you and your child.