First of all I must say you have made a great choice for your first cruise. RCI is our favorite cruise line, the Voyager class has the most activities on the high seas, and oh--to have a balcony--you will be forever spoiled for anything less.
For Brian and me, the aft balcony staterooms are our favorite. You can see the view off of both sides of the ship, and the sound of/view of the ships wake is a wonderful thing.
Now, I must confess that we have not yet travelled on a Voyager class ship! Our experience is limited to staterooms on the aft of the Radiance class ships as well as having booked aft staterooms on other lines.
I have been very, very curious about the aft end of this class of ship because of the outer super-structure that is there. Over the past half year I have been searching online for opinions/experiences/photos, and whenever we've been in port with these ships, I run around like a jerk trying to look and see what's what. Early in July we were on the Sovereign while she was in Nassau with the Voyager and the Mariner (one of the two of them was not on its regular itinerary due to the hurricane). I tried to learn what I could; what I was thinking was not promising: what is this architectural outer structure even there for? Grumble.
Then two weeks ago, I found a photo of one of these ships taken from a slightly-above vantage point. Brian and I studied this photo and decided that we may one day get an aft stateroom IF it was on the top/10th or 9th deck only---and only if it were one of the outboard staterooms. Well this sounds like exactly what you've got reserved!
Lastly, there is only one difference between a Cat D and a Cat E stateroom: simply less square footage in the center of the stateroom. In a Cat D, we've had a "3-cushion" sofa; in a Cat E, there was room only for a "chair and a half" or love-seat size of furniture. The bathrooms and closets and desk/chair area are exactly the same. The width of the stateroom is the same.
One last positive point for your Deck 9 stateroom is that there are staterooms above and below you: no public spaces with chairs draggin or music thumping.